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ABPath Dermatopathology study materials
ABPath Dermatopathology study resources

ABPath Dermatopathology practice questions

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Question 1Diagnostic dermatopathology and related clinical and laboratory knowledge

Which cell layer of the epidermis contains desmosomal attachments to neighboring keratinocytes and is the site of acantholysis in pemphigus vulgaris?

  1. Stratum basale
  2. Stratum granulosum
  3. Stratum corneum
  4. Stratum spinosum Correct answer.

Explanation

Suprabasal acantholysis in pemphigus vulgaris reflects autoantibodies against desmoglein 3, which is most concentrated in the lower spinous layer. Loss of desmosomal adhesion there produces the characteristic intraepidermal cleft just above the basal layer.

Question 2Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a pigmented lesion shows a proliferation of uniform melanocytes confined to the basal layer with increased single-cell density but no pagetoid spread or atypia. Which diagnosis is most appropriate?

  1. Melanoma in situ
  2. Junctional melanocytic nevus
  3. Lentigo maligna
  4. Solar lentigo Correct answer.

Explanation

Solar lentigo shows a linear basal proliferation of melanocytes without atypia, pagetoid scatter, or confluence, distinguishing it from melanoma in situ. The absence of cytologic atypia and architectural disorder supports a benign lentiginous process.

Question 3Diagnostic dermatopathology and related clinical and laboratory knowledge

Which immunodeposit pattern on direct immunofluorescence is classic for bullous pemphigoid?

  1. Intercellular IgG in the epidermis
  2. Linear IgG and C3 along the basement membrane zone Correct answer.
  3. Granular IgA in dermal papillae
  4. Fibrinogen in vessel walls

Explanation

Bullous pemphigoid is a subepidermal blistering disease with autoantibodies against hemidesmosomal antigens, producing linear IgG and C3 deposition along the basement membrane zone. This contrasts with the intercellular pattern of pemphigus and the granular papillary pattern of dermatitis herpetiformis.

Question 4Diagnostic dermatopathology and related clinical and laboratory knowledge

Histology of a chronic eczematous plaque shows epidermal spongiosis with intraepidermal vesiculation and a perivascular lymphocytic infiltrate. Which term best describes the intraepidermal fluid collection?

  1. Spongiosis Correct answer.
  2. Hyperkeratosis
  3. Ballooning degeneration
  4. Acantholysis

Explanation

Spongiosis refers to intercellular edema between keratinocytes that widens desmosomal spaces and may progress to intraepidermal vesicles, the hallmark of eczematous dermatitis. Acantholysis and ballooning degeneration are distinct mechanisms of intraepidermal clefting.

Question 5Diagnostic dermatopathology and related clinical and laboratory knowledge

A dermal nodule shows a proliferation of spindle cells arranged in a storiform pattern with scattered multinucleated giant cells and a sparse inflammatory infiltrate. Which diagnosis is favored?

  1. Nodular fasciitis
  2. Dermatofibrosarcoma protuberans
  3. Dermatofibroma Correct answer.
  4. Keloid

Explanation

Dermatofibroma is characterized by a storiform proliferation of spindle cells with entrapment of collagen bundles at the periphery and often giant cells. The storiform architecture with a relatively bland, well-circumscribed dermal process supports this diagnosis.

Question 6Diagnostic dermatopathology and related clinical and laboratory knowledge

Which feature most reliably distinguishes seborrheic keratosis from melanoma in situ on low-power examination?

  1. Presence of horn cysts and pseudohorn cysts Correct answer.
  2. Lymphocytic infiltrate
  3. Presence of melanin pigment
  4. Epidermal hyperplasia

Explanation

Horn cysts and pseudohorn cysts are architectural hallmarks of seborrheic keratosis and are not a feature of melanoma in situ. Pigment, epidermal hyperplasia, and inflammation can be seen in both processes and are less discriminating.

Question 7Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient has grouped vesicles on an erythematous base and a biopsy shows multinucleated keratinocytes with margination of chromatin. Which organism is most likely responsible?

  1. Molluscum contagiosum
  2. Herpes simplex virus Correct answer.
  3. Varicella-zoster virus
  4. Human papillomavirus

Explanation

Herpes simplex virus produces ballooning degeneration with multinucleated keratinocytes and margination of chromatin, often with ground-glass nuclei. Grouped vesicles on an erythematous base are the classic clinical correlate.

Question 8Diagnostic dermatopathology and related clinical and laboratory knowledge

In lichen planus, which histologic finding corresponds to the clinical Wickham striae?

  1. Dermal mucin deposition
  2. Subcorneal pustules
  3. Hypergranulosis with a band-like lymphocytic infiltrate at the dermoepidermal junction Correct answer.
  4. Parakeratosis with neutrophils in the stratum corneum

Explanation

Lichen planus shows a band-like lymphocytic infiltrate obscuring the dermoepidermal junction with hypergranulosis and irregular acanthosis, correlating with Wickham striae. These features reflect interface dermatitis with basal cell damage.

Question 9Diagnostic dermatopathology and related clinical and laboratory knowledge

A skin biopsy of a patient with chronic kidney disease shows extensive calcium deposition in the dermis with a foreign-body giant cell reaction. Which term describes this finding?

  1. Calcinosis cutis Correct answer.
  2. Pseudoxanthoma elasticum
  3. Osteoma cutis
  4. Gouty tophus

Explanation

Calcinosis cutis refers to deposition of calcium salts in the skin, often with a foreign-body giant cell reaction when deposits are large. It is associated with connective tissue disease and chronic renal failure.

Question 10Diagnostic dermatopathology and related clinical and laboratory knowledge

Which immunohistochemical marker is most useful in confirming a diagnosis of melanoma in a challenging spindle cell lesion?

  1. Cytokeratin
  2. CD34
  3. Desmin
  4. S100 and Melan-A Correct answer.

Explanation

Melanoma is confirmed with melanocytic markers such as S100, Melan-A, and HMB-45, which retain sensitivity in many spindle cell variants. Cytokeratin, CD34, and desmin mark other lineages and are used to exclude alternative diagnoses.

Question 11Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy shows parakeratosis, regular acanthosis with thinning over dermal papillae, and dilated capillaries in the papillary dermis. Which diagnosis is most likely?

  1. Psoriasis Correct answer.
  2. Tinea corporis
  3. Pityriasis rosea
  4. Lichen simplex chronicus

Explanation

Psoriasis shows parakeratosis, regular acanthosis with club-shaped rete ridges, thinning of the suprapapillary plate, and dilated papillary dermal capillaries. Neutrophils in the stratum corneum (Munro microabscesses) may also be present.

Question 12Diagnostic dermatopathology and related clinical and laboratory knowledge

A dermal infiltrate composed of atypical lymphocytes with epidermotropism and Pautrier microabscesses is most consistent with which diagnosis?

  1. Mycosis fungoides Correct answer.
  2. Chronic lymphocytic leukemia cutis
  3. Lymphomatoid papulosis
  4. Erythema multiforme

Explanation

Mycosis fungoides shows epidermotropic atypical T lymphocytes with Pautrier microabscesses, reflecting early cutaneous T-cell lymphoma. This distinguishes it from other lymphoid infiltrates and interface dermatitides.

Question 13Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic feature is characteristic of a hypertrophic scar rather than a keloid?

  1. Thick eosinophilic collagen bundles
  2. Dermal nodules with thick collagen
  3. Preserved dermal architecture with increased collagen Correct answer.
  4. Nodular myxoid areas

Explanation

Hypertrophic scars show increased collagen with preserved overall dermal architecture and tend to regress, whereas keloids extend beyond the original wound with thick, glassy collagen bundles. The distinction rests on architecture and clinical behavior.

Question 14Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient presents with a violaceous, indurated plaque on the shin and a biopsy shows dermal edema, extravasated erythrocytes, and a perivascular infiltrate with fibrin in vessel walls. Which diagnosis is most likely?

  1. Erythema nodosum
  2. Pigmented purpuric dermatosis
  3. Leukocytoclastic vasculitis Correct answer.
  4. Stasis dermatitis

Explanation

Leukocytoclastic vasculitis shows fibrinoid necrosis of vessel walls, neutrophilic infiltration with nuclear dust, and extravasated erythrocytes, producing palpable purpura. The presence of fibrin in vessel walls is a key diagnostic feature.

Question 15Diagnostic dermatopathology and related clinical and laboratory knowledge

Which structure is selectively lost in alopecia areata, producing the characteristic 'swarm of bees' peribulbar infiltrate?

  1. Sebaceous glands
  2. Eccrine glands
  3. Dermal papillae
  4. Hair follicle bulb Correct answer.

Explanation

Alopecia areata is a non-scarring alopecia with a lymphocytic infiltrate concentrated around the hair bulb, sometimes described as a 'swarm of bees'. The bulb is the immunologic target, leading to hair shedding without follicular destruction.

Question 16Diagnostic dermatopathology and related clinical and laboratory knowledge

Which molecular marker is characteristic of dermatofibrosarcoma protuberans and supports its diagnosis?

  1. TFE3 rearrangement
  2. MYC amplification
  3. COL1A1-PDGFB fusion Correct answer.
  4. EWSR1-FLI1 fusion

Explanation

Dermatofibrosarcoma protuberans is characterized by a t(17;22) translocation producing a COL1A1-PDGFB fusion, which drives PDGFB overexpression. This fusion distinguishes it from other spindle cell tumors and guides targeted therapy.

Question 17Diagnostic dermatopathology and related clinical and laboratory knowledge

A child develops multiple hypopigmented macules on the trunk, and a biopsy shows decreased melanin with normal melanocyte number. Which diagnosis is most likely?

  1. Pityriasis alba Correct answer.
  2. Vitiligo
  3. Pityriasis versicolor
  4. Tuberous sclerosis

Explanation

Pityriasis alba shows hypopigmentation with reduced melanin but normal melanocyte numbers, often with mild spongiosis. It is common in children and typically affects the face and trunk.

Question 18Diagnostic dermatopathology and related clinical and laboratory knowledge

Which finding on biopsy distinguishes a Spitz nevus from a conventional melanocytic nevus?

  1. Dermal fibrosis
  2. Nests of large spindle and epithelioid melanocytes with maturation Correct answer.
  3. Uniform small nevus cells in nests
  4. Presence of melanin pigment

Explanation

Spitz nevi contain large spindle and epithelioid melanocytes arranged in nests that mature with depth, a key feature separating them from conventional nevi and from melanoma. Kamino bodies may also be present.

Question 19Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient with a history of a tick bite develops an expanding erythematous plaque with a central clearing. Which histologic finding is most consistent with erythema migrans?

  1. Fibrinoid necrosis of vessel walls
  2. Interface dermatitis with a lymphocytic infiltrate and plasma cells Correct answer.
  3. Dermal mucin deposition
  4. Subcorneal pustules

Explanation

Erythema migrans shows a superficial and deep perivascular lymphocytic infiltrate with interface changes and often plasma cells, reflecting Borrelia infection. This pattern helps distinguish it from other inflammatory dermatoses.

Question 20Diagnostic dermatopathology and related clinical and laboratory knowledge

Which feature best characterizes the histology of a dermatofibroma at its periphery?

  1. Necrobiosis of collagen
  2. Grenz zone beneath the epidermis
  3. Honeycomb infiltration of subcutaneous fat
  4. Entrapment of collagen bundles by spindle cells Correct answer.

Explanation

At the periphery of a dermatofibroma, spindle cells entrap thick collagen bundles, producing a characteristic 'collagen trapping' pattern. This helps distinguish it from dermatofibrosarcoma protuberans, which infiltrates fat in a honeycomb pattern.

Question 21Diagnostic dermatopathology and related clinical and laboratory knowledge

Which type of amyloid is deposited in primary cutaneous amyloidosis?

  1. Beta-2 microglobulin
  2. AA amyloid derived from serum amyloid A
  3. Keratin-derived amyloid Correct answer.
  4. AL amyloid derived from immunoglobulin light chains

Explanation

Primary cutaneous amyloidosis, including macular and lichen amyloidosis, is composed of keratin-derived amyloid deposited in the papillary dermis. Systemic amyloidoses deposit AL or AA proteins and are distinguished by clinical context and staining.

Question 22Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a blistering eruption shows a subepidermal blister with neutrophils concentrated at the dermal papillary tips. Which diagnosis is most likely?

  1. Pemphigus vulgaris
  2. Epidermolysis bullosa acquisita
  3. Dermatitis herpetiformis Correct answer.
  4. Bullous pemphigoid

Explanation

Dermatitis herpetiformis shows subepidermal blisters with neutrophils clustered at the tips of dermal papillae, correlating with granular IgA deposition. This pattern is distinct from the linear basement membrane deposition of bullous pemphigoid.

Question 23Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic finding is most characteristic of granuloma annulare?

  1. Touton giant cells with lipid vacuoles
  2. Palisading granulomas around necrobiotic collagen Correct answer.
  3. Caseating granulomas
  4. Naked sarcoidal granulomas

Explanation

Granuloma annulare shows palisading histiocytes surrounding zones of necrobiotic collagen with mucin deposition in the dermis. This pattern distinguishes it from sarcoidal and infectious granulomas.

Question 24Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a violaceous plaque on the ear shows a proliferation of atypical cells with abundant eosinophilic cytoplasm and marked pleomorphism. Which diagnosis should be considered?

  1. Atypical fibroxanthoma Correct answer.
  2. Basal cell carcinoma
  3. Nodular melanoma
  4. Squamous cell carcinoma

Explanation

Atypical fibroxanthoma occurs on sun-exposed sites such as the ear and shows pleomorphic spindle and epithelioid cells with atypical mitoses but a relatively circumscribed dermal growth. It is a diagnosis of exclusion after ruling out melanoma and carcinoma.

Question 25Diagnostic dermatopathology and related clinical and laboratory knowledge

Which feature is characteristic of lichen sclerosus on histology?

  1. Thinning of the epidermis with a homogenized, sclerotic papillary dermis Correct answer.
  2. Dermal mucin with perivascular lymphocytes
  3. Subepidermal blister with eosinophils
  4. Regular acanthosis with parakeratosis

Explanation

Lichen sclerosus shows epidermal thinning with a band of homogenized, sclerotic collagen in the papillary dermis and a lymphocytic infiltrate below it. These changes produce the clinical appearance of white, crinkled plaques.

Question 26Diagnostic dermatopathology and related clinical and laboratory knowledge

A punch biopsy of an annular erythematous plaque on the trunk shows a superficial and deep perivascular and interstitial lymphocytic infiltrate with scattered eosinophils and prominent dermal mucin. Which diagnosis is best supported?

  1. Erythema multiforme
  2. Perniosis
  3. Gyrate erythema (erythema annulare centrifugum) Correct answer.
  4. Sweet syndrome

Explanation

Gyrate erythema characteristically shows a tight perivascular lymphocytic infiltrate, often described as a 'coat-sleeve' pattern, with variable dermal mucin and occasional eosinophils but no vasculitis. Sweet syndrome would show dense neutrophilic dermal infiltrate with edema.

Question 27Diagnostic dermatopathology and related clinical and laboratory knowledge

A skin biopsy shows a well-circumscribed dermal nodule of epithelioid cells with abundant pale cytoplasm, admixed lymphocytes, and scattered Touton giant cells. Which diagnosis is most likely?

  1. Granular cell tumor
  2. Xanthogranuloma Correct answer.
  3. Reticulohistiocytoma
  4. Dermatofibroma

Explanation

Juvenile xanthogranuloma is a non-Langerhans cell histiocytosis showing sheets of foamy to epithelioid histiocytes with Touton giant cells and a mixed inflammatory infiltrate. Touton giant cells are the key feature distinguishing this from other histiocytic lesions.

Question 28Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic finding best supports a diagnosis of pigmented purpuric dermatosis?

  1. Leukocytoclastic vasculitis with nuclear dust and neutrophils
  2. Extravasated erythrocytes with hemosiderin deposition and a perivascular lymphocytic infiltrate Correct answer.
  3. Lymphocytic vasculitis with fibrinoid necrosis of vessel walls
  4. Thrombotic occlusion of dermal vessels with fibrin thrombi

Explanation

Pigmented purpuric dermatoses show erythrocyte extravasation, hemosiderin-laden macrophages, and a superficial perivascular lymphocytic infiltrate without true vasculitis. Fibrinoid necrosis and thrombi are features of vasculitis or thrombotic disorders.

Question 29Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a pearly papule on the eyelid shows a cystic space lined by stratified squamous epithelium with keratinization and numerous adjacent small basaloid nests with peripheral palisading and retraction artifact. Which diagnosis is correct?

  1. Nodular basal cell carcinoma with cystic change Correct answer.
  2. Hidrocystoma
  3. Sebaceous adenoma
  4. Trichilemmal cyst

Explanation

Nodular basal cell carcinoma can undergo cystic degeneration, producing a keratin-filled space, but the diagnostic basaloid nests with peripheral palisading and clefting remain the key feature. Hidrocystomas lack basaloid nests.

Question 30Diagnostic dermatopathology and related clinical and laboratory knowledge

Which immunohistochemical staining pattern is most characteristic of a desmoplastic melanoma?

  1. Strong cytokeratin 5/6 positivity
  2. Diffuse CD34 positivity with STAT6 nuclear staining
  3. Diffuse strong Melan-A and HMB-45 positivity
  4. Diffuse strong S100 and SOX10 positivity with weak or absent Melan-A and HMB-45 Correct answer.

Explanation

Desmoplastic melanoma is typically strongly positive for S100 and SOX10 but frequently negative or only focally positive for melanocytic markers such as Melan-A and HMB-45. This pattern helps distinguish it from other spindle cell lesions.

Question 31Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy from a patient with a purpuric rash shows a superficial small-vessel vasculitis with fibrinoid necrosis and neutrophils. Direct immunofluorescence is most likely to demonstrate which finding?

  1. Granular IgA in the dermal papillary tips
  2. Linear IgG along the basement membrane
  3. Perivascular deposition of IgM and C3 Correct answer.
  4. Linear C3 along the basement membrane

Explanation

Leukocytoclastic vasculitis typically shows perivascular deposition of IgM and C3 in early lesions, reflecting immune complex deposition in vessel walls. Linear basement membrane staining is seen in bullous pemphigoid or Goodpasture-type patterns.

Question 32Diagnostic dermatopathology and related clinical and laboratory knowledge

Which feature is most characteristic of a neurofibroma on histology?

  1. Bland spindle cells with wavy nuclei embedded in a myxoid stroma with mast cells Correct answer.
  2. Dense hyalinized collagen bundles with sparse cells
  3. Spindle cells arranged in a storiform pattern with collagen trapping
  4. Epithelioid cells in nests with prominent nucleoli

Explanation

Neurofibromas consist of bland spindle cells with wavy, comma-shaped nuclei in a loose myxoid to collagenous stroma and often contain mast cells. The storiform pattern with collagen trapping describes dermatofibroma.

Question 33Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient with a history of a blistering eruption on sun-exposed skin has a biopsy showing a subepidermal blister with a predominantly neutrophilic infiltrate and linear deposition of IgG and C3 along the basement membrane on direct immunofluorescence. Which diagnosis is most likely?

  1. Pemphigus vulgaris
  2. Bullous pemphigoid Correct answer.
  3. Linear IgA bullous dermatosis
  4. Dermatitis herpetiformis

Explanation

Bullous pemphigoid is characterized by a subepidermal blister with eosinophils and neutrophils and linear IgG and C3 along the basement membrane on direct immunofluorescence. Pemphigus vulgaris would show intraepidermal acantholysis.

Question 34Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic finding is characteristic of a pilomatricoma?

  1. Nests of sebocytes with holocrine secretion
  2. Spindle cells in a myxoid stroma
  3. Ghost cells and basaloid cells with calcification Correct answer.
  4. Ductal structures lined by two cell layers

Explanation

Pilomatricoma shows islands of basaloid cells that abruptly transition into anucleate eosinophilic shadow (ghost) cells, often with calcification and foreign body giant cell reaction. Sebaceous differentiation and ductal structures are features of other adnexal tumors.

Question 35Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a slowly enlarging nodule on the scalp of an older adult shows a dermal proliferation of basaloid cells arranged in islands with a cribriform pattern and prominent basement membrane material. Which diagnosis is most likely?

  1. Syringoma
  2. Cylindroma Correct answer.
  3. Spiradenoma
  4. Trichoepithelioma

Explanation

Cylindroma is characterized by islands of basaloid cells arranged in a jigsaw-puzzle pattern with thick basement membrane material and ductal differentiation. Trichoepithelioma shows horn cysts and abortive hair follicles.

Question 36Diagnostic dermatopathology and related clinical and laboratory knowledge

Which feature distinguishes a malignant peripheral nerve sheath tumor from a benign neurofibroma?

  1. Presence of wavy nuclei
  2. Nuclear atypia with mitotic activity and necrosis Correct answer.
  3. Myxoid stroma
  4. Presence of mast cells

Explanation

Malignant peripheral nerve sheath tumors show nuclear atypia, increased mitotic activity, and necrosis, features absent in benign neurofibromas. Wavy nuclei, myxoid stroma, and mast cells can be seen in both and do not distinguish malignancy.

Question 37Diagnostic dermatopathology and related clinical and laboratory knowledge

A skin biopsy shows a dense dermal infiltrate of mononuclear cells with folded nuclei and abundant pale cytoplasm, with prominent epidermotropism but no spongiosis. Which immunohistochemical profile is most consistent with this lesion?

  1. CD56+, CD3-
  2. CD3+, CD4+, CD7- Correct answer.
  3. CD3+, CD8+, CD7+
  4. CD20+, CD79a+

Explanation

Mycosis fungoides is a mature T-cell lymphoma typically showing a CD4+ memory T-cell phenotype with loss of pan-T markers such as CD7. CD8 predominance or CD56 positivity suggests other lymphoma subtypes.

Question 38Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic feature is most characteristic of a dermatofibrosarcoma protuberans?

  1. Epithelioid cells with prominent nucleoli and frequent mitoses
  2. A well-circumscribed nodule with peripheral collagen trapping
  3. A proliferation of bland spindle cells with abundant myxoid stroma and no fat infiltration
  4. A storiform proliferation of spindle cells infiltrating subcutaneous fat in a honeycomb pattern Correct answer.

Explanation

Dermatofibrosarcoma protuberans shows a storiform spindle cell proliferation that infiltrates subcutaneous fat, producing a honeycomb or lace-like pattern. Collagen trapping at the periphery is characteristic of dermatofibroma.

Question 39Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a scaly plaque shows parakeratosis, hypogranulosis, and vacuolar alteration of the basal layer with necrotic keratinocytes. Which diagnosis is most likely?

  1. Lichen planus
  2. Discoid lupus erythematosus Correct answer.
  3. Pityriasis rosea
  4. Psoriasis

Explanation

Discoid lupus erythematosus shows interface dermatitis with vacuolar change, necrotic keratinocytes, and follicular plugging with periadnexal inflammation. Psoriasis shows regular acanthosis with neutrophils but no interface change.

Question 40Diagnostic dermatopathology and related clinical and laboratory knowledge

Which finding on histology is most characteristic of a keloid?

  1. Granulomatous inflammation with foreign body giant cells
  2. Thick eosinophilic collagen bundles in a haphazard array with few fibroblasts Correct answer.
  3. Myxoid stroma with scattered spindle cells
  4. Parallel collagen bundles with numerous fibroblasts

Explanation

Keloids show thick, glassy eosinophilic collagen bundles arranged haphazardly with relatively few fibroblasts, extending beyond the original wound. Hypertrophic scars show parallel collagen bundles with more numerous fibroblasts.

Question 41Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient presents with a violaceous, scaly plaque on the dorsal hand. Biopsy shows a dense band-like lymphocytic infiltrate obscuring the dermoepidermal junction with necrotic keratinocytes and pigment incontinence. Which diagnosis is most likely?

  1. Psoriasis
  2. Lichen planus Correct answer.
  3. Tinea corporis
  4. Eczema

Explanation

Lichen planus shows a band-like lymphocytic infiltrate obscuring the dermoepidermal junction with necrotic keratinocytes and pigment incontinence. Psoriasis and eczema lack the dense interface infiltrate.

Question 42Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a violaceous plaque on the ear shows a proliferation of atypical cells with abundant eosinophilic cytoplasm and prominent nucleoli, with a prominent lymphocytic infiltrate. Which diagnosis is most likely?

  1. Angiosarcoma
  2. Squamous cell carcinoma
  3. Kaposi sarcoma
  4. Atypical fibroxanthoma Correct answer.

Explanation

Atypical fibroxanthoma occurs on sun-damaged skin of the head and neck, especially the ear, and shows pleomorphic spindle and epithelioid cells with abundant eosinophilic cytoplasm and numerous mitoses. The lesion is superficial and lacks deep invasion.

Question 43Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic feature most reliably distinguishes a seborrheic keratosis from a melanoma in situ?

  1. Presence of melanin pigment
  2. Presence of epidermal hyperplasia
  3. Presence of horn cysts and pseudohorn cysts Correct answer.
  4. Presence of a perivascular lymphocytic infiltrate

Explanation

Horn cysts and pseudohorn cysts are characteristic of seborrheic keratosis and are not a feature of melanoma in situ. Melanin pigment and epidermal hyperplasia can occur in both lesions.

Question 44Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient has grouped vesicles on an erythematous base, and a biopsy shows multinucleated keratinocytes with margination of chromatin. Which diagnosis is most likely?

  1. Cytomegalovirus infection
  2. Herpes simplex virus infection Correct answer.
  3. Human papillomavirus infection
  4. Varicella-zoster virus infection

Explanation

Herpes simplex virus produces multinucleated keratinocytes with margination of chromatin, molding, and ground-glass nuclei. Varicella-zoster shows similar changes but typically has more prominent necrosis and affects a dermatome.

Question 45Diagnostic dermatopathology and related clinical and laboratory knowledge

A patient with a history of a tick bite develops an expanding erythematous plaque with central clearing. Which histologic finding is most characteristic?

  1. Interface dermatitis with necrotic keratinocytes
  2. A granulomatous infiltrate with caseation
  3. A dense neutrophilic infiltrate with abscess formation
  4. A superficial and deep perivascular lymphocytic infiltrate with plasma cells Correct answer.

Explanation

Erythema migrans shows a superficial and deep perivascular lymphocytic infiltrate often with plasma cells, reflecting Borrelia infection. Neutrophilic abscesses or caseating granulomas suggest other infections.

Question 46Diagnostic dermatopathology and related clinical and laboratory knowledge

A skin biopsy shows a dermal proliferation of spindle cells arranged in a storiform pattern with entrapment of subcutaneous fat at the deep margin. Which diagnosis is most likely?

  1. Dermatofibrosarcoma protuberans Correct answer.
  2. Benign fibrous histiocytoma
  3. Desmoplastic melanoma
  4. Nodular fasciitis

Explanation

Storiform spindle cells infiltrating subcutaneous fat in a honeycomb pattern are characteristic of dermatofibrosarcoma protuberans. Benign fibrous histiocytoma tends to remain confined to the dermis with peripheral collagen trapping.

Question 47Diagnostic dermatopathology and related clinical and laboratory knowledge

Which histologic pattern is the hallmark of a lichenoid tissue reaction?

  1. Diffuse interstitial infiltrate of histiocytes
  2. Neutrophilic infiltrate with leukocytoclasia
  3. Perivascular infiltrate with dermal eosinophils
  4. Band-like lymphocytic infiltrate obscuring the dermoepidermal junction Correct answer.

Explanation

Lichenoid reactions feature a dense band-like lymphocytic infiltrate hugging the dermoepidermal junction with basal damage. This pattern is seen in lichen planus and lichenoid drug eruptions.

Question 48Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of a solitary violaceous nodule shows a well-circumscribed dermal proliferation of uniform spindle cells with eosinophilic cytoplasm, numerous mast cells, and a grenz zone. Which diagnosis is most likely?

  1. Glomus tumor
  2. Dermatofibroma
  3. Leiomyoma Correct answer.
  4. Neurofibroma

Explanation

A well-circumscribed dermal nodule of uniform eosinophilic spindle cells with a grenz zone and mast cells is characteristic of a piloleiomyoma. Mast cells are often scattered among the smooth muscle bundles.

Question 49Diagnostic dermatopathology and related clinical and laboratory knowledge

Which immunohistochemical marker is most useful for confirming a diagnosis of Merkel cell carcinoma?

  1. S100 protein
  2. CD34
  3. Desmin
  4. Cytokeratin 20 with dot-like paranuclear positivity Correct answer.

Explanation

Merkel cell carcinoma characteristically shows cytokeratin 20 positivity in a dot-like paranuclear pattern. This helps distinguish it from other small blue cell tumors.

Question 50Diagnostic dermatopathology and related clinical and laboratory knowledge

A biopsy of an erythematous scaly plaque shows full-thickness epidermal atypia with loss of maturation and numerous mitoses above the basal layer. Which diagnosis is most likely?

  1. Psoriasis
  2. Actinic keratosis
  3. Seborrheic keratosis
  4. Squamous cell carcinoma in situ Correct answer.

Explanation

Full-thickness epidermal atypia with loss of maturation and suprabasal mitoses defines squamous cell carcinoma in situ. Actinic keratosis shows atypia confined to the lower epidermis.