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FVH Pathologie practice questions

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Question 1Veterinary pathology, specimens and interpretation

A dog's liver shows firm, tan, wedge-shaped areas with preserved tissue architecture on histology - no nuclei in hepatocytes but cellular outlines remain. This pattern represents which type of necrosis?

  1. Liquefactive necrosis
  2. Caseous necrosis
  3. Coagulative necrosis Correct answer.
  4. Fat necrosis

Explanation

Coagulative necrosis preserves tissue architecture (ghost outlines) with loss of nuclei (karyolysis/pyknosis/karyorrhexis) - typical of ischemic injury in solid organs except brain. Liquefactive occurs in CNS and abscesses; caseous in mycobacterial/fungal infections; fat necrosis in pancreatitis/adipose trauma.

Question 2Veterinary pathology, specimens and interpretation

Which feature BEST distinguishes apoptosis from necrosis histologically?

  1. Neutrophilic inflammation surrounds apoptotic cells
  2. Apoptosis shows individual cell shrinkage with intact membranes and no inflammation Correct answer.
  3. Apoptosis always occurs in sheets of cells
  4. Necrosis preserves cell membranes

Explanation

Apoptosis classically involves individual-cell shrinkage, condensed chromatin and membrane-bound fragments that can be cleared with limited inflammation. Necrosis commonly involves membrane disruption and an inflammatory response. The contrast is a useful pattern rather than a statement that secondary changes or impaired clearance can never modify the appearance.

Question 3Veterinary pathology, specimens and interpretation

Granulomatous inflammation is characterized by which predominant cell type?

  1. Neutrophils and fibrin
  2. Lymphocytes only
  3. Epithelioid macrophages and multinucleated giant cells Correct answer.
  4. Eosinophils and basophils

Explanation

Granulomas consist of epithelioid macrophages (activated, abundant eosinophilic cytoplasm) often fused into multinucleated giant cells (Langhans, foreign body, or Touton), frequently with a lymphocyte cuff. Seen with mycobacteria, systemic fungi, FIP, and foreign bodies.

Question 4Veterinary necropsy, postmortem change and specimen integrity

During the post-mortem examination of a bovine carcass 36 hours after death, greenish-black discoloration is observed on the serosa of the intestines and abdominal wall adjacent to the cecum. Histopathology reveals no inflammatory reaction or cellular necrosis associated with the pigment. What is the most accurate pathological interpretation of this gross finding?

  1. Pseudomelanosis caused by bacterial hydrogen sulfide reacting with hemoglobin iron from lysed erythrocytes Correct answer.
  2. Ante-mortem hemorrhagic-necrotizing enteritis with localized tissue gangrene
  3. Melanosis maculosa congenital deposition in the visceral peritoneum
  4. Bile imbibition resulting from post-mortem leakage of biliary salts across the gall bladder wall

Explanation

Pseudomelanosis is a common post-mortem artifact characterized by greenish-black or blue-black discoloration of abdominal viscera and the abdominal wall. It occurs when post-mortem saprophytic bacteria produce hydrogen sulfide (H2S), which reacts with iron from hemolyzed erythrocytes to form insoluble iron sulfide (FeS). Because it is a post-mortem change, there is no associated inflammatory cellular infiltrate or ante-mortem vascular response on histopathology.

Question 5Veterinary necropsy, postmortem change and specimen integrity

Which biochemical process is directly responsible for the onset and subsequent resolution of rigor mortis in mammalian carcasses following death?

  1. Accumulation of intracellular calcium causing persistent troponin activation without ATP depletion, followed by autolytic myofibril synthesis
  2. Exhaustion of intracellular ATP preventing actin-myosin cross-bridge dissociation, followed by resolution via enzymatic autolysis of myofibrillar proteins Correct answer.
  3. Excessive lactic acid neutralization preventing actin contraction, followed by putrefactive bacterial invasion
  4. Rapid post-mortem dephosphorylation of myosin light chains leading to permanent cross-linking until putrefaction occurs

Explanation

Rigor mortis (post-mortem rigidity) develops when glycogen and creatine phosphate stores are exhausted, leading to complete depletion of ATP. Because ATP binding is required to dissociate myosin heads from actin filaments, muscles remain locked in a contracted state. Rigor mortis resolves after several hours to days as endogenous proteolytic lysosomal enzymes and saprophytic bacteria degrade the structural myofibrillar proteins (autolysis).

Question 6Veterinary necropsy, postmortem change and specimen integrity

At post-mortem examination of a 4-month-old piglet, a segment of small intestine is invaginated into an adjacent distal segment. Which morphological feature definitively distinguishes a true ante-mortem pathological intussusception from an agonal (post-mortem) invagination?

  1. The invaginated segment easily slides out upon gentle traction without any mucosal resistance
  2. Absence of mesenteric vessel traction or luminal contents within the intussuscipiens
  3. Complete lack of serosal fibrin deposition, congestion, or edema in both intestinal layers
  4. Intense venous congestion, fibrin exudation, edema, and mural necrosis with resistance to manual reduction Correct answer.

Explanation

True ante-mortem intussusception involves mesenteric entrapment, leading to severe venous stasis, mural edema, thrombosis, ischemic necrosis, and fibrinous serositis that firmly binds the intussusceptum to the intussuscipiens, making manual reduction difficult or impossible. In contrast, agonal invaginations occur during the irregular hyperperistalsis of dying, lack vascular compromise, fibrin, or inflammation, and reduce easily with gentle traction.

Question 7Microscopic patterns, tissue injury and special stains

A histopathological examination of an organ demonstrates preservation of the basic cellular architecture and tissue outlines for several days, but the cells exhibit loss of nuclei (karyolysis), hypereosinophilia of the cytoplasm, and structural 'tombstone' outlines. What specific morphological pattern of necrosis is described?

  1. Caseous necrosis
  2. Liquefactive necrosis
  3. Coagulative necrosis Correct answer.
  4. Fibrinoid necrosis

Explanation

Coagulative necrosis is the classic hallmark of hypoxic/ischemic tissue death (infarction) in all solid organs except the brain. The acidic environment denatures not only structural proteins but also endogenous lysosomal enzymes, preventing immediate proteolysis. As a result, the dead cells retain their basic microscopic outline and tissue architecture ('tombstone' appearance) with intense cytoplasmic eosinophilia and loss of nuclei (pyknosis, karyorrhexis, karyolysis) until phagocytosed by infiltrating leukocytes.

Question 8Microscopic patterns, tissue injury and special stains

During histopathological examination of a granulomatous lymphadenitis in a steer with suspected mycobacterial infection, multinucleated giant cells are identified. Which morphological feature distinguishes a Langhans-type giant cell from a foreign body-type giant cell?

  1. Langhans giant cells have a central lipid ring with peripheral cytoplasm containing phagocytosed hair shafts
  2. Langhans giant cells are derived from neoplastic T-lymphocytes whereas foreign body giant cells derive from mast cells
  3. Langhans giant cells have only 2 to 3 central nuclei and contain abundant periodic acid-Schiff (PAS)-negative crystals
  4. Langhans giant cells possess nuclei arranged in a peripheral horseshoe- or ring-like pattern at the cell periphery, whereas foreign body giant cells have nuclei randomly scattered throughout the cytoplasm Correct answer.

Explanation

Langhans-type multinucleated giant cells, characteristically found in tuberculous granulomas and other chronic infectious granulomatous diseases, are formed by the fusion of activated epithelioid macrophages. They display nuclei arranged in a peripheral arc, semicircle, or horseshoe pattern around a central eosinophilic cytoplasm. In contrast, foreign body giant cells display haphazardly and randomly distributed nuclei throughout the center and cytoplasm.

Question 9Microscopic patterns, tissue injury and special stains

A 6-month-old feedlot steer presents with acute respiratory distress and dies. At necropsy, the cranioventral lung lobes are firm, consolidated, dark red-purple, and sink in formalin. Histopathology reveals the lumens of bronchi, bronchioles, and adjacent alveoli packed with degenerate neutrophils, fibrin, and necrotic cellular debris. What is the fundamental morphological pattern of pneumonia?

  1. Suppurative / fibrinosuppurative bronchopneumonia Correct answer.
  2. Diffuse interstitial pneumonia
  3. Embolic hematogenous pneumonia
  4. Granulomatous pneumonia

Explanation

Bronchopneumonia is characterized aerogenously by cranioventral consolidation. Infectious agents (such as Pasteurella multocida, Trueperella pyogenes, or Mannheimia haemolytica) enter via the airway, causing inflammation that originates at the bronchiolar-alveolar junction and spills into adjacent alveoli. The exudate consists predominantly of neutrophils, fibrin, and necrotic epithelial debris filling the airways and alveolar spaces.

Question 10Immunophenotyping, clonality and molecular assay interpretation

In diagnostic veterinary immunohistochemistry (IHC), which combination of intermediate filament antibodies is standard to distinguish carcinomas, sarcomas, and muscular tumors?

  1. CD3 for carcinomas, CD20 for sarcomas, and CD18 for myogenic tumors
  2. Cytokeratins (e.g. AE1/AE3) for epithelial tumors (carcinomas), Vimentin for mesenchymal tumors (sarcomas), and Desmin for myogenic tumors (muscle neoplasms) Correct answer.
  3. S100 for carcinomas, Neurofilament for sarcomas, and IBA1 for myogenic tumors
  4. Chromogranin A for carcinomas, Synaptophysin for sarcomas, and Calcitonin for myogenic tumors

Explanation

The listed intermediate-filament markers can contribute to a lineage panel: cytokeratins support epithelial differentiation, vimentin supports a broad mesenchymal interpretation and desmin contributes to muscle assessment. Expression can overlap or be lost. Morphology, species validation and additional markers are needed rather than treating one stain as definitive proof of every tumor origin.

Question 11Immunophenotyping, clonality and molecular assay interpretation

Which immunohistochemical antibody panel is standard in veterinary diagnostic pathology for immunophenotyping canine and feline lymphomas into T-cell versus B-cell lineages on formalin-fixed, paraffin-embedded (FFPE) tissue sections?

  1. Melan-A (T-cell) and PNL2 (B-cell)
  2. Vimentin (T-cell) and Cytokeratin (B-cell)
  3. CD3 (pan T-cell marker) and CD20 / CD79a / PAX5 (pan B-cell markers) Correct answer.
  4. Thyroglobulin (T-cell) and Calcitonin (B-cell)

Explanation

For lymphoma immunophenotyping in FFPE tissues, CD3 (specifically anti-CD3 epsilon) is the universal, robust pan-T-lymphocyte marker. For B-lymphocytes, CD20 (a transmembrane phosphoprotein), CD79a (part of the B-cell receptor complex), and PAX5 (a B-cell lineage-specific transcription factor) are the standard, reliable diagnostic markers. PAX5 is particularly valuable when anti-CD20 antibodies are altered by previous rituximab-like therapy or when plasma cell differentiation occurs.

Question 12Immunophenotyping, clonality and molecular assay interpretation

An undifferentiated, highly pleomorphic, amelanotic oral mucosal neoplasm in a dog is negative for Cytokeratin AE1/AE3 and CD3/CD20. Which panel of diagnostic IHC markers is most sensitive and specific for confirming a diagnosis of malignant melanoma?

  1. Desmin, Smooth Muscle Actin (SMA), and Myogenin
  2. Thyroglobulin, Calcitonin, and TTF-1
  3. Chromogranin A, Synaptophysin, and CD56
  4. Melan-A (MART-1), PNL2, S100, and Tyrosinase-related proteins (TRP-1/2) Correct answer.

Explanation

Melanomas (particularly amelanotic variants) can mimic carcinomas, sarcomas, or round cell tumors. The standard veterinary melanoma cocktail/panel includes Melan-A (MART-1), PNL2 (highly specific for canine melanomas), S100 protein (highly sensitive, though less specific as it also stains neural and myoepithelial tissues), and Tyrosinase / TRP-1/2. Combining Melan-A, PNL2, and S100 achieves >95-98% diagnostic sensitivity for canine malignant melanomas.

Question 13Cytology, blood findings, effusions and clinical chemistry

During fine-needle aspiration (FNA) cytology of a cutaneous mass in a dog, which cytological criteria reliably distinguish epithelial neoplasms from mesenchymal (spindle cell) neoplasms?

  1. Epithelial cells exfoliate exclusively as naked nuclei in a background of lipid droplets
  2. Epithelial cells are always discrete single round cells lacking all intercellular junctions, whereas mesenchymal cells form rigid glandular tubules
  3. Epithelial cells have abundant metachromatic cytoplasmic granules that obscure the nucleus, whereas mesenchymal cells have eccentric clock-face chromatin
  4. Epithelial cells exfoliate in tight cohesive clusters, sheets, or acinar structures with distinct cell borders and round to polygonal shapes, whereas mesenchymal cells exfoliate individually or in loose aggregates as spindle- to fusiform-shaped cells with indistinct cytoplasmic margins and tail-like cytoplasmic projections Correct answer.

Explanation

On diagnostic cytology, neoplastic cell lineages exhibit classic exfoliation patterns. Epithelial neoplasms (carcinomas/adenomas) have strong intercellular junctions (desmosomes, tight junctions), causing them to exfoliate in high cellularity as cohesive clusters, sheets, cords, or acinar/tubular groups with well-defined cell borders and polygonal-to-cuboidal shapes. Mesenchymal neoplasms (sarcomas) have lower exfoliation cellularity, exfoliate individually or in loose wispy clusters, and display elongated, fusiform, or stellate morphologies with indistinct cytoplasmic borders trailing into fibrillar matrix.

Question 14Cytology, blood findings, effusions and clinical chemistry

A fine-needle aspirate of a solitary dermal mass on the flank of a 5-year-old Boxer dog reveals high cellularity with round cells containing abundant purple-to-violet cytoplasmic granules that partially obscure the central round nucleus. Eosinophils and scattered reactive fibroblasts are present in the background. What is the cytological diagnosis?

  1. Cutaneous mast cell tumor Correct answer.
  2. Cutaneous melanoma
  3. Cutaneous histiocytoma
  4. Transmissible venereal tumor

Explanation

Canine mast cell tumors (MCTs) are classic round cell neoplasms. On Romanowsky-type stains (Diff-Quik, Giemsa, Wright), mast cell cytoplasm contains numerous distinct metachromatic purple-magenta granules containing histamine, heparin, chymase, and tryptase. The release of chemotactic factors (e.g. eosinophil chemotactic factor of anaphylaxis) characteristically attracts concurrent eosinophils, and fibroplasia is commonly observed in the background.

Question 15Cytology, blood findings, effusions and clinical chemistry

A 10-month-old Golden Retriever presents with a rapidly growing, button-like, alopecic, ulcerated nodule on the pinna. FNA cytology reveals a monomorphic population of discrete round cells with pale, lightly basophilic cytoplasm, round-to-indented eccentric nuclei, fine chromatin, and an infiltration of small mature lymphocytes at the margins. What is the most likely diagnosis?

  1. B-cell lymphoblastic lymphoma
  2. Canine cutaneous histiocytoma Correct answer.
  3. Squamous cell carcinoma
  4. Amelanotic melanoma

Explanation

Canine cutaneous histiocytoma is a benign neoplasm of epidermal Langerhans cells (dendritic cells) common in dogs under 3 years of age, especially on the head, ears, and limbs. Cytologically, it presents as discrete round cells with moderate pale-staining cytoplasm, round-to-reniform (cleaved) eccentric nuclei, and delicate chromatin. Spontaneous regression occurs within weeks due to a cell-mediated (CD8+ T-lymphocyte) immune response, reflected cytologically and histologically by an influx of small lymphocytes.

Question 16Veterinary tumor classification, grading and prognostic limits

In canine mammary tumor classification, what defines a 'complex' mammary tumor (e.g. complex adenoma or complex carcinoma) versus a 'simple' or 'mixed' mammary tumor?

  1. Complex tumors originate exclusively from multiple mammary glands simultaneously
  2. Complex tumors contain necrotic tissue and bacterial infection
  3. Complex tumors consist of both neoplastic luminal epithelial cells AND proliferating myoepithelial cells, whereas simple tumors consist purely of a single epithelial or myoepithelial cell type, and mixed tumors contain epithelial and myoepithelial elements accompanied by heterologous mesenchymal tissues (bone, cartilage, or fat) Correct answer.
  4. Complex tumors have metastasized to the brain

Explanation

Under the Davis-Thompson / Goldschmidt classification for canine mammary tumors: 1) Simple tumors consist of a single neoplastic cell population (luminal epithelium or myoepithelium); 2) Complex tumors are composed of proliferating luminal epithelial cells alongside a distinct proliferated myoepithelial component (often producing stellate/spindle cells in a myxoid background); and 3) Mixed tumors (benign mixed tumor / carcinosarcoma) contain epithelial, myoepithelial, and true ectopic/heterologous mesenchymal matrices (cartilage, bone, or adipose tissue formed through myoepithelial metaplasia).

Question 17Veterinary tumor classification, grading and prognostic limits

In the WHO/REAL classification of canine lymphomas, what are the characteristic immunophenotypic, cytological, and clinical features of Canine T-zone Lymphoma (TZL)?

  1. Indolent, low-grade T-cell lymphoma originating from the paracortex, characterized by small-to-intermediate CD3+ CD45- (loss of CD45 / leukocyte common antigen expression) neoplastic lymphocytes with low mitotic rate, long survival times (often >2-3 years), and frequent involvement of peripheral blood and spleen Correct answer.
  2. High-grade, rapidly fatal B-cell lymphoma with CD20+ CD79a+ immunophenotype and prominent starry-sky appearance
  3. Acute lymphoblastic leukemia with universal bone marrow aplasia and high mortality in under 48 hours
  4. Primary cutaneous epitheliotropic lymphoma caused by feline leukemia virus

Explanation

Canine T-zone lymphoma (TZL) is an indolent form of lymphoma originating from T-cells in the paracortical T-zone of lymph nodes (frequent in Golden Retrievers). The neoplastic cells are small to intermediate in size, CD3+ T-cells, and uniquely display loss of CD45 (CD45-negative on flow cytometry). Because of its indolent nature and very low mitotic rate, dogs with TZL often exhibit generalized lymphadenopathy and lymphocytosis but enjoy prolonged survival (median >2–3 years) without aggressive cytotoxic chemotherapy.

Question 18Veterinary tumor classification, grading and prognostic limits

What is the definitive, pathognomonic histopathological criterion required to confirm a diagnosis of Osteosarcoma in a bone biopsy?

  1. Presence of multi-nucleated osteoclasts lining normal Haversian canals
  2. Direct production of unmineralized or mineralized osteoid matrix (eosinophilic, amorphous, homogeneous matrix) by malignant mesenchymal (osteoblastic) cells Correct answer.
  3. Pure chondroid matrix production without any osteoid or bone formation
  4. Infiltration of mature plasma cells with Russell bodies

Explanation

Osteosarcoma (OSA) is a primary malignant bone neoplasm. Regardless of whether the tumor contains abundant cartilage (chondroblastic OSA), fibroblasts (fibroblastic OSA), blood-filled vascular spaces (telangiectatic OSA), or giant cells, the sine qua non (definitive diagnostic criterion) is the direct production of osteoid matrix (woven bone / osteoid seams) directly by the malignant neoplastic mesenchymal cells without an intervening chondroid or fibrous precursor scaffold.

Question 19Toxic and metabolic lesions across veterinary species

Cattle and horses grazing on pastures infested with Senecio jacobaea (Ragwort) or Echium vulgare develop chronic weight loss, icterus, and hepatic failure. Histopathological evaluation of the liver demonstrates the pathognomonic classic triad of Pyrrolizidine Alkaloid Toxicosis. What constitutes this classic triad?

  1. Kupffer cell hyperplasia, iron overload, and amyloid deposition
  2. Massive hepatic steatosis, centrilobular congestion, and lipofuscinosis
  3. Megalocytosis (giant hepatocytes with enlarged nuclei), biliary ductular hyperplasia, and bridging periportal fibrosis Correct answer.
  4. Granulomatous cholangiohepatitis with intracellular acid-fast bacilli

Explanation

Pyrrolizidine alkaloids (PAs, found in Senecio, Crotalaria, Cynoglossum, Echium) are metabolized by hepatic cytochrome P450 enzymes into reactive dehydropyrrolizidine (pyrrole) derivatives. Pyrroles are potent alkylating antimitotic agents that crosslink DNA: hepatocytes can synthesize DNA and duplicate chromosomes but cannot undergo mitosis/cytokinesis. This leads to marked hepatocyte enlargement (megalocytosis / karyomegaly). As megalocytes die, secondary biliary ductular hyperplasia and bridging periportal fibrosis develop, forming the pathognomonic diagnostic triad of chronic PA toxicosis.

Question 20Toxic and metabolic lesions across veterinary species

A 3-year-old cat dies of acute oliguric renal failure 24 hours after suspected toxic ingestion. At necropsy, the kidneys are swollen and pale with a wet, bulging cortex. Histopathological examination reveals acute proximal tubular epithelial necrosis with tubular lumens packed with pale yellow, radiating, rosette-like prismatic crystals that display brilliant birefringence under polarized light. What toxin is responsible?

  1. Ethylene glycol (antifreeze), whose metabolite oxalic acid precipitates with calcium to form calcium oxalate monohydrate crystals in renal tubules Correct answer.
  2. Gentamicin aminoglycoside toxicosis (causing myeloid bodies in lysosomes without crystals)
  3. Melamine and cyanuric acid (forming circular golden-brown spherulites)
  4. Cholecalciferol (vitamin D3) toxicosis (causing metastatic calcium phosphate mineralization)

Explanation

Ethylene glycol (antifreeze) is metabolized by hepatic alcohol dehydrogenase into glycoaldehyde, glycolic acid, and finally oxalic acid (oxalate). Oxalic acid binds ionized calcium to form insoluble calcium oxalate monohydrate crystals. These precipitate extensively within the lumens and cytoplasm of renal proximal tubular epithelial cells, causing physical obstruction, acute tubular necrosis, severe metabolic acidosis, and hypocalcemia. The crystals appear as pale yellow, radiating rosettes/prisms that shine brilliantly (birefringence) under cross-polarized light.

Question 21Toxic and metabolic lesions across veterinary species

A flock of sheep in Switzerland accidentally receives a mineral mix formulated for cattle with high copper content. Several sheep develop acute icterus, dark brown/port-wine colored urine (hemoglobinuria), and death. At necropsy, the carcasses are severely jaundiced, the liver is enlarged and friable, and the kidneys are enlarged, dark blue-black to 'gunmetal' in color. What is the pathogenesis of this acute hemolytic crisis?

  1. Copper binds to acetylcholine receptors causing immediate neuromuscular blockade
  2. Copper accumulates silently in hepatic lysosomes over weeks to months until storage capacity is exceeded; a stressful event triggers sudden massive release of free copper into the bloodstream, causing oxidative lysis of erythrocyte membranes (acute intravascular hemolysis), methemoglobinemia, and hemoglobinuric acute tubular nephrosis Correct answer.
  3. Copper inhibits bone marrow erythropoiesis causing pure aplastic anemia
  4. Copper induces acute immune-mediated thrombocytopenia

Explanation

Sheep are exquisitely sensitive to copper toxicity because their biliary excretion of copper is very low. Ingested copper accumulates silently inside hepatic lysosomes over weeks. When hepatocyte storage capacity is saturated (or triggered by a stress event), hepatocytes undergo necrosis, causing sudden massive release of copper into the circulation. Blood copper levels spike, causing severe oxidative damage to red blood cell membranes, massive acute intravascular hemolysis, methemoglobinemia, and hemoglobinuria. The kidneys become dark blue-black ('gunmetal kidneys') due to extensive hemoglobin accumulation and hemoglobinuric nephrosis.

Question 22Veterinary necropsy, postmortem change and specimen integrity

An 18-year-old Arabian gelding is euthanized due to acute, unrelenting colic. Necropsy reveals a sharply demarcated, 3-meter loop of jejunum that is dark reddish-black, thickened, and friable. A spherical, 6-cm pedunculated fatty mass is wrapped around the mesenteric root of the affected loop. What is the definitive morphological diagnosis?

  1. Hemorrhagic infarction secondary to strangulating pedunculated lipoma Correct answer.
  2. Non-strangulating thromboembolic colic secondary to Strongylus vulgaris migration
  3. Pelvic flexure enterolithiasis with secondary pressure necrosis
  4. Idiopathic muscular hypertrophy of the ileum with secondary intussusception

Explanation

Pedunculated mesenteric lipomas are benign adipose tumors of older equids that arise from the mesenteric fat. As they grow on elongated fibrovascular stalks, they can wrap around loops of small intestine (or small colon), obstructing venous drainage first and then arterial supply. This causes severe venous stasis, mural edema, and hemorrhagic infarction of the entrapped intestinal loop.

Question 23Veterinary necropsy, postmortem change and specimen integrity

A 10-year-old German Shepherd dog collapses and dies acutely. Necropsy reveals 400 mL of dark, unclotted blood within the pericardial sac, compressing the ventricles (cardiac tamponade). A 3-cm multinodular, dark red, friable mass is situated in the wall of the right atrium. What is the most likely diagnosis?

  1. Aortic body chemodectoma
  2. Myxomatous mitral valve rupture
  3. Right atrial hemangiosarcoma Correct answer.
  4. Infective vegetative endocarditis of the tricuspid valve

Explanation

Hemangiosarcoma is a malignant neoplasm of endothelial cell origin with a strong predilection for the right atrium/auricle and the spleen in large breed dogs, especially German Shepherds and Golden Retrievers. Rupture of a right atrial hemangiosarcoma leads to acute hemopericardium, resulting in fatal cardiac tamponade and cardiogenic shock.

Question 24Veterinary necropsy, postmortem change and specimen integrity

Which gross pathological feature reliably differentiates chronic myxomatous mitral valvular degeneration (endocardiosis) from active bacterial endocarditis in a canine heart?

  1. Endocardiosis exhibits friable, yellow-to-gray vegetative fibrinonecrotic plaques with superficial bacterial colonies
  2. Endocardiosis causes extensive transmural myocardial infarction with diffuse suppurative pericarditis
  3. Endocardiosis causes severe subvalvular aortic ring calcification with aortic rupture
  4. Endocardiosis presents as smooth, glistening, nodular thickenings of the valve leaflets without ulceration or thrombotic vegetations Correct answer.

Explanation

Myxomatous valvular degeneration (endocardiosis) is an age-related degenerative disease characterized by nodular, smooth, glistening, white-to-pearly thickenings of the free edges of the atrioventricular valve leaflets (most commonly the mitral valve), lacking inflammation, necrosis, or bacterial vegetations. Bacterial endocarditis, conversely, causes rough, friable, irregular, yellow-red 'cauliflower-like' fibrinonecrotic vegetations containing bacterial colonies.

Question 25Veterinary necropsy, postmortem change and specimen integrity

At post-mortem examination of a cow with congestive heart failure secondary to chronic traumatic reticuloperitonitis, the liver is enlarged with rounded borders. On cross-section, there is a striking reticular pattern of dark red centrilobular zones interspersed with pale yellowish-tan periportal parenchyma ('nutmeg liver'). What is the underlying pathogenesis of this lesion?

  1. Severe chronic passive hepatic congestion with centrilobular sinusoidal dilatation, hypoxia-induced atrophy/necrosis of hepatocytes, and fatty degeneration of periportal hepatocytes Correct answer.
  2. Acute massive hepatic necrosis (hepatosis dietetica) induced by selenium and vitamin E deficiency
  3. Severe biliary hyperplasia with periportal fibrosis resulting from chronic Fasciola hepatica migration
  4. Diffuse amyloid deposition within the space of Disse causing pressure atrophy of periportal hepatocytes

Explanation

Chronic passive hepatic congestion (nutmeg liver) results from elevated central venous pressure (such as in right-sided congestive heart failure). Impaired venous outflow distends the central veins and centrilobular sinusoids, causing hypoxia, atrophy, and necrosis of centrilobular hepatocytes (appearing dark red). In contrast, periportal hepatocytes receive relatively oxygenated blood and survive, but undergo hypoxia-induced fatty change (appearing pale yellow-tan).

Question 26Veterinary necropsy, postmortem change and specimen integrity

During the necropsy of a 6-month-old Yorkshire Terrier puppy with stunted growth and hepatic encephalopathy, a single large anomalous vessel is identified connecting the portal vein directly to the caudal vena cava, bypassing the hepatic parenchyma. The liver is markedly microhepatic. What is the definitive diagnosis and histological correlate?

  1. Acquired multiple portosystemic shunts with severe nodular regenerative hyperplasia
  2. Congenital extrahepatic portosystemic shunt with hepatic arteriolar proliferation, small/absent intrahepatic portal veins, and lobular atrophy Correct answer.
  3. Primary portal vein thrombosis with cavernomatous collateral transformation
  4. Arteriovenous fistula of the celiac axis with marked sinusoidal peliosis hepatis

Explanation

Congenital extrahepatic portosystemic shunts (PSS) are common in small and toy breed dogs (such as Yorkshire Terriers). The single anomalous vessel diverts portal blood directly into the systemic circulation (caudal vena cava or azygos vein). This leads to microhepatica. Histopathologically, the liver exhibits small or absent portal vein branches, compensatory proliferation of small tortuous hepatic arterioles, and generalized lobular atrophy with lipofuscin accumulation.

Question 27Veterinary necropsy, postmortem change and specimen integrity

A 9-year-old cat with hypertrophic cardiomyopathy develops acute paraparesis and cold, pulseless hindlimbs. At necropsy, both kidneys exhibit sharply demarcated, wedge-shaped, pale tan cortical areas with the base at the renal capsule and apex pointing toward the medulla, surrounded by a hyperemic rim. What is the precise morphological diagnosis?

  1. Acute ascending suppurative pyelonephritis
  2. Renal cortical amyloidosis with secondary nephrotic syndrome
  3. Acute to subacute renal cortical coagulative infarcts secondary to thromboembolism Correct answer.
  4. Bilateral hydronephrosis secondary to distal ureteral obstruction

Explanation

Renal infarcts are classic examples of coagulative necrosis resulting from sudden occlusion of an interlobular or arcuate artery (end-arteries lacking anastomoses). Because renal interlobular and arcuate arteries supply cone-shaped segments of the cortex, the resulting infarct is wedge-shaped, with the apex pointing to the occluded vessel in the corticomedullary junction and the base at the capsular surface. Thromboemboli originating from a left atrial thrombus in feline HCM frequently lodge in the renal arteries as well as the aortic bifurcation ('saddle thrombus').

Question 28Veterinary necropsy, postmortem change and specimen integrity

A 3-week-old lamb is found dead in spring. Necropsy reveals distinct bilateral, symmetrical, pale, chalky-white streaks within the semitendinosus and semimembranosus skeletal muscles, as well as subendocardial white patches within the left ventricular myocardium. What is the fundamental biochemical mechanism underlying this condition?

  1. Severe intracellular hypocalcemia preventing actin-myosin dissociation during cardiac systole
  2. Excess dietary sulfur competing with thiamine absorption in the rumen
  3. Clostridium chauvoei alpha-toxin mediated phospholipase C activation and gas gangrene
  4. Deficiency of vitamin E and/or selenium leading to loss of cellular antioxidant protection and free radical-induced peroxidative myofiber necrosis Correct answer.

Explanation

White muscle disease (enzootic nutritional muscular dystrophy) in young ruminants results from deficiency of selenium (a cofactor for glutathione peroxidase) and/or vitamin E (a lipid-soluble antioxidant). This deficiency leads to unimpeded reactive oxygen species (ROS) generation and free radical-mediated lipid peroxidation of cellular and mitochondrial membranes. The influx of extracellular calcium into damaged myofibers produces hypercontraction, coagulative necrosis, and mineralization (chalky white streaks).

Question 29Veterinary necropsy, postmortem change and specimen integrity

At the necropsy of an obese, 8-year-old Miniature Schnauzer with acute collapse, the peripancreatic mesentery and omentum display numerous firm, opaque, chalky-white nodules surrounded by a hyperemic zone. The pancreas is swollen, dark red to black, and friable. What pathological process is responsible for the chalky nodules in the adipose tissue?

  1. Enzymatic fat necrosis (saponification) caused by pancreatic lipase release hydrolyzing triglycerides into free fatty acids that complex with calcium Correct answer.
  2. Metastatic mineralization secondary to chronic renal failure and hyperphosphatemia
  3. Granulomatous steatitis induced by Mycobacterium avium subsp. paratuberculosis infection
  4. Ischemic gangrene secondary to cranial mesenteric arterial thrombosis

Explanation

In acute necrotizing pancreatitis, premature activation of pancreatic proenzymes causes autodigestion of pancreatic parenchyma and peripancreatic fat. Pancreatic lipases hydrolyze neutral fats (triglycerides) within adipocytes into glycerol and free fatty acids. The released fatty acids bind ionized calcium (Ca2+) to form insoluble, chalky-white calcium soaps (saponification / enzymatic fat necrosis).

Question 30Veterinary necropsy, postmortem change and specimen integrity

During the necropsy of a 4-month-old Bulldog puppy with a history of severe cyanosis and exercise intolerance, a congenital cardiac anomaly is identified. Which tetrad of cardiovascular lesions defines the classic Tetralogy of Fallot?

  1. Persistent right aortic arch, left ventricular hypoplasia, tricuspid atresia, and patent ductus arteriosus
  2. Ventricular septal defect (VSD), pulmonic stenosis, overriding (dextropositioned) aorta, and secondary right ventricular concentric hypertrophy Correct answer.
  3. Atrial septal defect (ASD), mitral valve dysplasia, aortic stenosis, and left ventricular eccentric hypertrophy
  4. Endocardial fibroelastosis, cor triatriatum dexter, pulmonic valve insufficiency, and left atrial rupture

Explanation

Tetralogy of Fallot is a classic congenital cyanotic cardiac defect resulting from malalignment of the embryonic conotruncal septum. It comprises: 1) Pulmonic stenosis (infundibular or valvular); 2) High ventricular septal defect (VSD); 3) Overriding (dextropositioned) aorta straddling the septal defect; and 4) Compensatory right ventricular hypertrophy developing secondary to the outflow obstruction and high right ventricular pressures.

Question 31Veterinary necropsy, postmortem change and specimen integrity

A dairy farm in Switzerland experiences multiple late-term abortions. During the diagnostic necropsy of an aborted bovine fetus and placenta, the placental intercotyledonary areas are severely thickened, leathery, and covered by a yellow-brown exudate, with necrotic, cupped cotyledons. The fetal abomasum contains turbid, yellow-white fluid with dense clusters of fungal hyphae branching at 45 degrees. What is the most likely etiologic diagnosis?

  1. Brucella abortus placentitis
  2. Neospora caninum protozoal abortion
  3. Chlamydia abortus enzootic abortion
  4. Mycotic placentitis and abomasitis caused by Aspergillus fumigatus Correct answer.

Explanation

Mycotic abortion in cattle, primarily caused by Aspergillus fumigatus or Mucorales (Mortierella, Absidia), presents with severe necrotizing placentitis featuring leather-like thickening of intercotyledonary areas and necrotic, enlarged cotyledons. The fetus ingests amniotic fluid containing fungal elements, leading to mycotic abomasitis and occasionally focal ringworm-like skin lesions. Histopathology demonstrates dichotomously branching (45°), parallel-walled, septate hyphae (Aspergillus) with special fungal stains (Grocott's methenamine silver or PAS).

Question 32Microscopic patterns, tissue injury and special stains

What is the key histopathological hallmark of the acute (exudative) phase of Diffuse Alveolar Damage (DAD) in acute respiratory distress syndrome (ARDS) across animal species?

  1. Extensive peribronchiolar lymphoid hyperplasia (cuffing) with intraluminal foreign body giant cells
  2. Formation of prominent intra-alveolar eosinophilic hyaline membranes along denuded alveolar basements, accompanied by capillary congestion and alveolar edema Correct answer.
  3. Diffuse intra-alveolar calcification with secondary osseous metaplasia of alveolar septa
  4. Widespread squamous metaplasia of type II pneumocytes with complete obliteration of the bronchial cartilage

Explanation

Diffuse Alveolar Damage (DAD) is the morphological correlate of ARDS. In the acute exudative phase (first 1–7 days), acute necrosis of type I pneumocytes and capillary endothelial cells leads to high-permeability pulmonary edema. Plasma proteins, especially fibrinogen, leak into the alveolar spaces and combine with necrotic cellular debris and surfactant remnants to form dense, waxy, eosinophilic bands lining the denuded alveolar ducts and septa, known as hyaline membranes. This is followed in the subacute/proliferative phase by type II pneumocyte hyperplasia.

Question 33Microscopic patterns, tissue injury and special stains

Adult beef cattle moved from dry summer pasture to lush, green, high-protein autumn pasture in Switzerland develop acute respiratory distress (Fog Fever / Acute Bovine Pulmonary Emphysema and Edema). What is the chemical pathogenesis of this condition?

  1. Excess dietary nitrates are converted into nitrites, inducing methemoglobinemia and pulmonary vascular spasm
  2. Excess dietary molybdenum induces secondary copper deficiency and capillary endothelial degeneration
  3. Ruminal fermentation of dietary L-tryptophan generates 3-methylindole (3-MI), which is metabolized by pulmonary Clara cells/cytochrome P450 into toxic intermediates that selectively necrotize type I pneumocytes Correct answer.
  4. Inhalation of fungal aflatoxins on wet grass leads to immediate type I hypersensitivity and mast cell degranulation in terminal bronchioles

Explanation

Acute Bovine Pulmonary Edema and Emphysema (ABPEE / Fog Fever) occurs when cattle abruptly transition to lush pastures rich in the amino acid L-tryptophan. Ruminal microbes convert L-tryptophan into 3-methylindole (3-MI). 3-MI is absorbed into the bloodstream and carried to the lungs, where Club cells (Clara cells) and type II pneumocytes express cytochrome P450 monooxygenases (CYP2F) that bioactivate 3-MI into reactive electrophilic intermediates (e.g. 3-methyleneindolenine). This selectively destroys type I pneumocytes and Club cells, causing acute exudative interstitial pneumonia, severe edema, interstitial emphysema, and subsequent type II pneumocyte hyperplasia.

Question 34Microscopic patterns, tissue injury and special stains

A 10-year-old Shar-Pei dog with recurrent pyrexia develops severe proteinuria and renal failure. Histopathological examination of the kidney reveals extracellular, amorphous, eosinophilic, acellular material expanding the glomerular mesangium and capillary tufts. Which special histochemical stain and optical condition provide the definitive microscopic diagnosis for amyloidosis?

  1. Periodic acid-Schiff (PAS) showing intense magenta staining resistant to diastase under brightfield illumination
  2. Toluidine blue producing bright metachromatic purple staining of the deposits under phase-contrast microscopy
  3. Masson's trichrome demonstrating bright blue fibrillar staining under standard polarized light
  4. Congo red stain demonstrating salmon-pink coloration under standard brightfield illumination and characteristic apple-green birefringence under cross-polarized light Correct answer.

Explanation

Amyloid consists of non-branching antiparallel beta-pleated sheet fibrils. Congo red is the gold standard histochemical stain for amyloid: under standard brightfield microscopy, amyloid deposits stain pale orange-red to salmon-pink. When viewed under cross-polarized light, the aligned dye molecules within the beta-pleated sheets produce pathognomonic bright apple-green birefringence (dichroism).

Question 35Microscopic patterns, tissue injury and special stains

In feline membranous glomerulonephropathy (MGN), which ultrastructural and histopathological finding on Jones' periodic acid-methenamine silver (PAM) stain is considered pathognomonic?

  1. Thickened glomerular basement membranes with regular subepithelial projections ('spikes') protruding around electron-dense subepithelial immune complexes Correct answer.
  2. Diffuse crescent formation within Bowman's space composed of parietal epithelial cells and fibrin
  3. Severe mesangial nodular sclerosis with Kimmelstiel-Wilson nodules
  4. Segmental necrotizing fibrinous thrombi within the afferent arteriole with loss of podocytes

Explanation

Membranous glomerulonephropathy (MGN) is an immune complex-mediated glomerulopathy characterized by the deposition of subepithelial immune complexes along the outer (epithelial) aspect of the glomerular basement membrane (GBM). On Jones' silver stain (PAM), the basement membrane material grows between and around the non-argyrophilic immune deposits, creating classic black argyrophilic 'spikes' and a 'comb-like' or 'moth-eaten' appearance.

Question 36Microscopic patterns, tissue injury and special stains

A 4-year-old Bedlington Terrier presents with chronic hepatitis and progressive icterus. Histopathology reveals centrilobular (zone 3) to midzonal hepatocyte necrosis, marked lipofuscinosis, and bridging fibrosis. Which special histochemical stain is required to confirm abnormal intracellular copper accumulation, and what is the typical color of the copper granules?

  1. Prussian blue (Perls') stain demonstrating bright turquoise blue granules in Kupffer cells
  2. Rhodanine (or Rubeanic acid) stain demonstrating bright red-brown to orange granular cytoplasmic deposits in hepatocytes Correct answer.
  3. Von Kossa stain demonstrating dark brown-black calcium precipitates
  4. Hall's stain demonstrating brilliant emerald green bile pigment deposits

Explanation

Canine copper-associated hepatopathy (e.g. due to COMMD1 gene mutation in Bedlington Terriers or ATP7A/ATP7B mutations in Labradors) causes toxic copper accumulation primarily in zone 3 (centrilobular) hepatocytes. The gold standard stains for copper are Rhodanine (producing bright red-brown to orange-red cytoplasmic granules in lysosomes) or Rubeanic acid (producing dark green-black granules).

Question 37Microscopic patterns, tissue injury and special stains

To differentiate between severe hepatic lipidosis (steatosis) and steroid-induced glycogen hepatopathy in a canine liver biopsy on formalin-fixed, paraffin-embedded (FFPE) sections, which staining protocol is most appropriate?

  1. Standard H&E is sufficient because lipid remains in paraffin sections while glycogen is completely dissolved by xylene
  2. Oil Red O on routine FFPE sections will stain neutral lipids bright red because paraffin embedding preserves intracellular triglycerides
  3. Periodic acid-Schiff (PAS) stain with and without diastase pre-treatment: glycogen is PAS-positive and diastase-sensitive (digested), whereas lipid vacuoles appear as clear unstained spaces on FFPE sections Correct answer.
  4. Gram stain to detect intracellular bacterial fermentation products

Explanation

During routine FFPE processing, organic solvents (alcohols and xylenes) dissolve intracellular neutral lipid droplets, leaving round, sharply punched-out empty vacuoles in the cytoplasm. Glycogen accumulation (e.g. from glucocorticoids / Cushing's) produces pale, feathery, indistinct cytoplasmic vacuolation. On FFPE sections, glycogen is PAS-positive (magenta) and diastase-sensitive (completely removed after alpha-amylase/diastase digestion). To stain neutral lipid directly with Oil Red O or Sudan Black, frozen (cryostat) sections of fresh/unprocessed tissue must be used.

Question 38Microscopic patterns, tissue injury and special stains

A skin biopsy from a dog with suspected discoid lupus erythematosus (DLE) is evaluated. What is the defining microscopic pattern of 'interface dermatitis' observed at the dermal-epidermal junction?

  1. Subcorneal pustules filled with intact acantholytic keratinocytes and non-degenerate eosinophils
  2. Massive nodular to diffuse infiltration of eosinophils and flame figures throughout the deep subcutis
  3. Marked follicular infundibular hyperkeratosis with intraluminal Demodex canis mites
  4. Vacuolar degeneration of basal keratinocytes, apoptotic basal keratinocytes (Civatte bodies), and a continuous band-like (lichenoid) lymphoplasmacytic infiltrate hugging the basement membrane zone with pigmentary incontinence Correct answer.

Explanation

Interface dermatitis is the histopathological hallmark of autoimmune and cytotoxic dermatoses such as Lupus Erythematosus (systemic or discoid) and Erythema Multiforme. It is characterized by basal cell hydropic degeneration (vacuolation), individual apoptotic keratinocytes (Civatte / colloid bodies) at the dermal-epidermal junction, obscuration of the basement membrane by a band-like (lichenoid) or perivascular lymphocytic infiltrate, and pigmentary incontinence (melanin dropped into the superficial dermis and engulfed by melanophages).

Question 39Microscopic patterns, tissue injury and special stains

What is the primary immunological target and microscopic hallmark of Pemphigus Foliaceus in dogs and horses?

  1. Autoantibodies against desmocollin-1 (or desmoglein-1) leading to loss of intercellular desmosomal adhesion (acantholysis) and subcorneal/intragranular pustules containing intact neutrophils and rounded acantholytic keratinocytes Correct answer.
  2. Autoantibodies targeting type VII collagen leading to subepidermal clefting and bullae formation in the lamina lucida
  3. Deposition of IgA immune complexes in the dermal papillae forming subepidermal microabscesses
  4. Direct cytotoxic T-cell destruction of hair follicle bulge stem cells causing non-inflammatory mural folliculitis

Explanation

Pemphigus foliaceus is the most common autoimmune skin disease in domestic animals (dogs, cats, horses). Autoantibodies (primarily IgG) bind to desmocollin-1 (in dogs) or desmoglein-1 (in horses/humans), which are essential cadherin adhesion molecules in desmosomes. Disruption of these desmosomes causes acantholysis (detachment of epidermal keratinocytes from one another). The classic histopathology is large subcorneal or intragranular pustules packed with non-degenerate neutrophils, variable eosinophils, and rafts of rounded, deeply eosinophilic acantholytic cells.

Question 40Microscopic patterns, tissue injury and special stains

A 1-year-old heifer presents with blindness, head pressing, opisthotonos, and convulsions. At necropsy, when the freshly sliced cerebral cortex is examined under a 365-nm ultraviolet (Wood's) lamp, brilliant autofluorescence is observed along the cerebrocortical grey matter. Histopathology reveals laminar cortical necrosis with neuronal necrosis and status spongiosus. What is the definitive diagnosis?

  1. Bovine Spongiform Encephalopathy (BSE)
  2. Rabies encephalitis
  3. Polioencephalomalacia (cerebrocortical necrosis, CCN) Correct answer.
  4. Thrombotic meningoencephalitis (TME) caused by Histophilus somni

Explanation

Polioencephalomalacia (PEM / cerebrocortical necrosis) in ruminants is caused by thiamine (vitamin B1) deficiency (e.g. due to ruminal thiaminases from amprolium, bracken fern, or grain overload) or high dietary sulfur intake. The energy depletion selectively damages the cerebral cortex. Macroscopically, necrotic cortical tissue exhibits autofluorescence under UV light (365 nm) due to lipopigment-laden lipid breakdown products (ceroid/lipofuscin) in macrophages. Microscopically, it presents as laminar neuronal necrosis, spongiosis of deep cortical layers, and astrocytosis.

Question 41Microscopic patterns, tissue injury and special stains

A horse fed moldy home-grown corn develops severe ataxia, head pressing, blindness, and sudden death. Necropsy reveals soft, cavitated, gelatinous, yellow-to-brown areas of liquefactive necrosis strictly confined to the subcortical white matter of the cerebral hemispheres. What toxin and fungal species are responsible for Equine Leukoencephalomalacia (ELEM)?

  1. Ergot alkaloids produced by Claviceps purpurea
  2. Swainsonine produced by Oxytropis species
  3. Ochratoxin A produced by Penicillium verrucosum
  4. Fumonisin B1 produced by Fusarium verticillioides (Fusarium moniliforme) Correct answer.

Explanation

Equine Leukoencephalomalacia (ELEM / 'moldy corn poisoning') is caused by the mycotoxin Fumonisin B1, produced by the fungus Fusarium verticillioides (formerly F. moniliforme) on corn. Fumonisin B1 inhibits the enzyme ceramide synthase (sphingosine N-acyltransferase), leading to accumulation of toxic sphinganine and sphingosine, disruption of endothelial sphingolipid metabolism, and vascular damage. In horses, this causes selective liquefactive necrosis of the subcortical cerebral white matter (leukoencephalomalacia) with hemorrhage, edema, and malacia.

Question 42Microscopic patterns, tissue injury and special stains

What is the classic histopathological triad that characterizes non-suppurative viral encephalitides in domestic animals (e.g. Borna disease, West Nile virus, Equine Infectious Anemia)?

  1. Perivascular lymphocytic/histiocytic cuffing, microglial nodules (gliosis), and neuronal degeneration with neuronophagia Correct answer.
  2. Massive purulent neutrophilic leptomeningitis, choroid plexus microabscesses, and ependymal calcification
  3. Extensive spongiosis of myelin sheaths with marked eosinophilic perivascular infiltrates
  4. Severe amyloid plaque deposition, neurofibrillary tangles, and vascular hyalinosis

Explanation

Viral encephalitides in domestic mammals are universally characterized by a non-suppurative inflammatory pattern. This comprises: 1) Perivascular cuffing (dense accumulation of lymphocytes, plasma cells, and histiocytes within Virchow-Robin spaces); 2) Gliosis with focal clusters of activated microglia (microglial nodules); and 3) Neuronal chromatolysis, necrosis, and neuronophagia (macrophages/microglia actively engulfing necrotic neurons).

Question 43Microscopic patterns, tissue injury and special stains

What is the primary underlying pathological defect in the development of Osteochondrosis Dissecans (OCD) in growing pigs, horses, and large breed dogs?

  1. Metastatic bacterial embolization into the synovial membrane causing purulent chondritis
  2. Immune-mediated destruction of type II collagen in the articular surface
  3. Focal failure of endochondral ossification leading to necrosis of retained epiphyseal cartilage, cleft formation within the necrotic cartilage, and secondary detachment of a dissecting cartilaginous flap Correct answer.
  4. Excessive osteoclastic bone resorption induced by secondary hyperparathyroidism

Explanation

Osteochondrosis is a developmental orthopedic disease resulting from a focal disturbance of endochondral ossification. Necrosis of cartilage canal blood vessels in growing epiphyseal cartilage prevents normal chondrocyte differentiation, calcification, and replacement by bone (osteochondrosis latens/manifesta). The retained ischemic cartilage is biomechanically weak. Under mechanical load, clefts and microfractures form through the necrotic cartilage (osteochondrosis dissecans / OCD), leading to a dissecting cartilage flap and exposure of subchondral bone.

Question 44Microscopic patterns, tissue injury and special stains

A 9-year-old cat with end-stage chronic kidney disease develops marked swelling and pliability of the mandible and maxilla ('rubber jaw'). Histopathology of the jaw bones reveals widespread osteoclastic bone resorption and replacement of trabecular bone by dense, cellular, fibrous connective tissue. What is the diagnosis and pathophysiology?

  1. Primary osteosarcoma of the mandible with paraneoplastic hypercalcemia
  2. Hypertrophic osteodystrophy (HOD) caused by canine distemper virus infection in bone
  3. Osteomalacia resulting from dietary vitamin D toxicity
  4. Fibrous osteodystrophy secondary to renal hyperparathyroidism (impaired renal phosphate excretion leading to hyperphosphatemia, decreased calcitriol, hypocalcemia, and continuous excessive PTH secretion) Correct answer.

Explanation

Fibrous osteodystrophy is a metabolic bone disease characterized by osteoclastic resorption of cortical and cancellous bone and its replacement by proliferating fibrous connective tissue. In renal secondary hyperparathyroidism (common in dogs/cats with chronic kidney disease), loss of functional nephrons leads to phosphate retention (hyperphosphatemia) and reduced synthesis of 1,25-dihydroxyvitamin D3 (calcitriol). The resulting hypocalcemia stimulates the parathyroid glands to chronically hypersecrete parathyroid hormone (PTH), driving generalized osteoclastic resorption.

Question 45Microscopic patterns, tissue injury and special stains

Which histological hallmark differentiates Rickets in growing animals from Osteomalacia in skeletally mature adult animals?

  1. Rickets is characterized by failure of mineralization of both physeal cartilage and osteoid, leading to enlargement/widening of the growth plates, whereas Osteomalacia affects only remodeling bone (unmineralized osteoid seams on trabeculae) because the growth plates are already closed Correct answer.
  2. Osteomalacia affects the growth plates (physis), while Rickets affects only the articular surfaces
  3. Rickets features severe narrowing of the physis with complete absence of osteoid
  4. Rickets produces extensive osteosarcoma transformation, whereas Osteomalacia causes chondroma formation

Explanation

Both Rickets (in young, growing animals) and Osteomalacia (in skeletally mature animals with closed physes) are caused by deficiency of vitamin D or phosphorus, leading to defective mineralization of bone matrix. In Rickets, because endochondral ossification is active, there is failure of provisional calcification of physeal cartilage. As a result, chondrocytes in the hypertrophic zone fail to degenerate, causing the growth plates to become markedly thickened, irregular, and flared (rachitic rosary). In adults with Osteomalacia, the physes are closed, so the defect manifests as wide seams of unmineralized osteoid on remodeling trabecular and cortical bone.

Question 46Microscopic patterns, tissue injury and special stains

A 2-year-old domestic shorthair cat presents with fever, weight loss, and multiple small nodular lesions along mesenteric vessels, liver, and serosa. Histopathological evaluation reveals pyogranulomas centered on small- to medium-sized veins and venules, characterized by central necrosis with degenerate neutrophils, surrounded by a broad zone of macrophages, lymphocytes, and plasma cells. What is the definitive diagnosis and vascular lesion?

  1. Toxoplasma gondii systemic tachyzoite embolism
  2. Feline Infectious Peritonitis (FIP) with pyogranulomatous phlebitis / perivasculitis Correct answer.
  3. Mycobacterium bovis primary lymphangitis
  4. Systemic necrotizing polyarteritis nodosa

Explanation

Feline Infectious Peritonitis (FIP), caused by mutated Feline Coronavirus (FIPV) replicating within monocytes/macrophages, is fundamentally an immune complex-mediated (type III hypersensitivity) and macrophage-driven disease. The pathognomonic vascular lesion is a pyogranulomatous phlebitis and periphlebitis (inflammation centered on veins and venules). Histopathologically, the perivascular lesions consist of central neutrophils, necrosis, and fibrin surrounded by epithelioid macrophages, lymphocytes, and abundant plasma cells (B-cell humoral activation).

Question 47Microscopic patterns, tissue injury and special stains

Which histopathological distinction accurately differentiates Mönckeberg's medial calcification from true atherosclerosis in domestic animals?

  1. Mönckeberg's medial calcification is caused by cytomegalovirus infection, whereas atherosclerosis is caused by Vitamin E deficiency
  2. Mönckeberg's medial calcification occurs only in veins, whereas atherosclerosis occurs exclusively in pulmonary capillaries
  3. Mönckeberg's medial calcification involves mineralization of the tunica media of muscular and elastic arteries without intimal proliferation or luminal obstruction, whereas atherosclerosis is characterized by lipid-laden intimal plaques (atheromas) with foamy macrophages and fibrous caps that narrow the vascular lumen Correct answer.
  4. Mönckeberg's medial calcification causes extensive intimal fibrinoid necrosis, whereas atherosclerosis affects only the adventitia

Explanation

Mönckeberg's medial calcification (medial arterial sclerosis) is characterized by ring-like, dystrophic or metastatic calcification within the elastic fibers of the tunica media of muscular and elastic arteries (common in aging animals, cattle with calcinogenic plant toxicosis, or chronic uremia). Importantly, it does not involve the tunica intima, does not produce lipid plaques, and does not occlude the lumen. In contrast, atherosclerosis is characterized by lipid and cholesterol accumulation in the tunica intima and inner media with foamy macrophages, smooth muscle proliferation, and luminal stenosis (seen in hypothyroid or hypercholesterolemic dogs).

Question 48Microscopic patterns, tissue injury and special stains

In feline gastrointestinal pathology, which histopathological and immunohistochemical criteria are most reliable for differentiating low-grade (small cell) alimentary T-cell lymphoma from severe chronic lymphoplasmacytic enteritis (IBD)?

  1. IBD is characterized by transmural T-cell destruction of muscularis propria, whereas small cell lymphoma is restricted strictly to the lamina propria
  2. Small cell lymphoma always contains large atypical mitotic figures (>10 per HPF) with extensive geographic necrosis, whereas IBD lacks lymphocytes
  3. IBD shows exclusively CD20+ B-cells with monoclonal PARR, whereas small cell lymphoma shows multinucleated histiocytic giant cells
  4. Small cell alimentary lymphoma is characterized by dense, monomorphic sheets of small CD3+ T-lymphocytes showing prominent epitheliotropism (forming intraepithelial plaques and nests extending into the villus tip and crypts), villous architectural distortion, infiltration into the muscularis mucosae/submucosa, and clonal T-cell receptor gamma (TCRG) rearrangement on PARR, whereas IBD displays a heterogeneous mixture of lymphocytes, plasma cells, eosinophils, and polyclonal PARR Correct answer.

Explanation

Monomorphic infiltrates, abnormal architecture, epitheliotropism and a compatible immunophenotype can support small-cell intestinal lymphoma in the appropriate feline case. Clonality testing adds evidence but does not alone establish malignancy, and inflammatory disease can also show restricted patterns. Interpret the complete tissue, controls, clinical course and sampling limits instead of treating clonal versus polyclonal as an infallible binary distinction.

Question 49Microscopic patterns, tissue injury and special stains

A 5-year-old dog from southern Europe presents with chronic generalized lymphadenopathy, exfoliative dermatitis, and proteinuria. A lymph node biopsy reveals diffuse infiltration of the medullary cords by plasma cells and macrophages packed with tiny (2-4 μm), round-to-oval intracellular organisms with a distinct nucleus and a perpendicular rod-like kinetoplast. What is the diagnosis?

  1. Leishmania infantum (amastigote stage) Correct answer.
  2. Blastomyces dermatitidis
  3. Histoplasma capsulatum
  4. Cryptococcus neoformans

Explanation

Canine leishmaniosis (Leishmania infantum) is an important vector-borne zoonosis (transmitted by phlebotomine sandflies). Within host macrophages (histiocytes), the parasite exists in its non-flagellated amastigote form. Microscopically, amastigotes measure 2–4 μm and are identified inside macrophages by possessing both a round basophilic nucleus and a distinct, smaller, rod-shaped or bar-shaped, perpendicularly oriented kinetoplast (specialized mitochondrial DNA structure), which definitively distinguishes them from Histoplasma or Toxoplasma.

Question 50Microscopic patterns, tissue injury and special stains

What is the primary histopathological consequence of sustained severe intraocular pressure elevation (glaucoma) on the retina and optic nerve head in domestic animals?

  1. Hyperplasia of the retinal pigment epithelium and hypertrophy of the rod and cone photoreceptor layer
  2. Selective necrosis and atrophy of the retinal ganglion cell layer and nerve fiber layer, followed by posterior cupping (excavation) of the optic disc and diffuse retinal atrophy Correct answer.
  3. Intraretinal osseous metaplasia with complete absence of optic nerve involvement
  4. Severe thickening of the inner plexiform layer due to eosinophilic infiltration

Explanation

Glaucoma is characterized by elevated intraocular pressure resulting from impaired aqueous humor outflow. The mechanical compression and axonal transport obstruction selectively cause early apoptosis and atrophy of the retinal ganglion cells and degeneration of their axons in the nerve fiber layer. Progressively, this leads to posterior bowing/cupping of the optic nerve head (lamina cribrosa) and diffuse inner retinal atrophy, culminating in full-thickness retinal thinning.