I85PREP
⌘ K
Egyptian Board Nephrology - independent subject foundations study materials
Egyptian Board Nephrology - independent subject foundations study resources

Egyptian Board Nephrology - independent subject foundations practice questions

One question at a time. Build your Egyptian Board Nephrology - independent subject foundations understanding.

First 50 of 190 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Renal structure and transport

A newborn infant presents at birth with massive proteinuria, profound generalized anasarca, and severe hypoalbuminemia. Genetic analysis confirms a homozygous mutation in the NPHS1 gene located on chromosome 19q13. Which structural podocyte slit diaphragm protein is directly encoded by this gene?

  1. Nephrin, a transmembrane cell adhesion protein of the immunoglobulin superfamily Correct answer.
  2. Podocin, an integral membrane hairpin protein localized to lipid rafts
  3. CD2-associated protein, an adaptor connecting the slit diaphragm to actin
  4. Alpha-actinin-4, an actin-filament cross-linking protein of the podocyte cytoskeleton

Explanation

NPHS1 encodes nephrin, a transmembrane protein of the immunoglobulin superfamily that forms the core homophilic and heterophilic zipper-like filtration units of the podocyte slit diaphragm. Mutations in NPHS1 cause Finnish-type congenital nephrotic syndrome, characterized by massive antenatal and neonatal proteinuria.

Question 2Renal structure and transport

A 58-year-old patient with diabetic kidney disease and hypertension is initiated on enalapril. A follow-up serum creatinine reveals a rise from 1.3 mg/dL to 1.5 mg/dL with stable blood pressure. Which hemodynamic alteration at the glomerulus explains this acute, anticipated physiological response?

  1. Afferent arteriolar constriction mediated by endothelin-1 activation
  2. Efferent arteriolar vasodilation reducing intraglomerular capillary hydrostatic pressure Correct answer.
  3. Afferent arteriolar vasodilation increasing renal plasma flow without affecting pressure
  4. Efferent arteriolar vasoconstriction increasing filtration fraction and capillary pressure

Explanation

Angiotensin II preferentially constricts the postglomerular efferent arteriole to maintain intraglomerular hydraulic pressure (Pgc). By inhibiting angiotensin II synthesis, ACE inhibitors cause efferent arteriolar vasodilation, lowering Pgc and causing a modest, reversible hemodynamic decline in GFR up to 30%.

Question 3Renal structure and transport

An adult male with type 2 diabetes and chronic kidney disease is prescribed empagliflozin. In which specific anatomical segment of the nephron does this pharmacological agent exert its primary therapeutic action?

  1. Distal convoluted tubule luminal apical membrane
  2. Thick ascending limb of the loop of Henle basolateral membrane
  3. Early proximal convoluted tubule (S1 and S2 segments) luminal brush border Correct answer.
  4. Cortical collecting duct principal cell apical membrane

Explanation

Sodium-glucose cotransporter 2 (SGLT2) is a high-capacity, low-affinity transporter located on the apical brush border membrane of the early proximal convoluted tubule (S1 and S2 segments). SGLT2 is responsible for reabsorbing approximately 90% of filtered glucose coupled to sodium.

Question 4Renal structure and transport

A patient with acute angle-closure glaucoma receives intravenous acetazolamide. Examination of the urine confirms marked alkalinization and bicarbonaturia. Through which specific transport mechanism does acetazolamide impair renal bicarbonate reabsorption?

  1. Inhibition of the basolateral sodium-bicarbonate cotransporter NBCe1
  2. Blockade of the apical pendrin chloride-bicarbonate exchanger in intercalated cells
  3. Direct antagonism of apical sodium-hydrogen exchanger 3 (NHE3) phosphorylation
  4. Inhibition of luminal carbonic anhydrase IV and intracellular carbonic anhydrase II Correct answer.

Explanation

Acetazolamide is a potent reversible inhibitor of both apical brush-border carbonic anhydrase IV and intracellular carbonic anhydrase II in the proximal convoluted tubule. This prevents the dehydration of luminal carbonic acid and the subsequent intracellular generation of hydrogen and bicarbonate ions, producing self-limited bicarbonaturia.

Question 5Renal structure and transport

A 62-year-old woman with decompensated heart failure is administered intravenous furosemide. Furosemide induces rapid natriuresis and calciuresis by binding to which specific tubular transport complex?

  1. Apical Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle Correct answer.
  2. Basolateral Na/K-ATPase pump in the medullary collecting duct
  3. Apical Na-Cl cotransporter (NCCT) in the distal convoluted tubule
  4. Apical Epithelial Sodium Channel (ENaC) in the cortical collecting duct

Explanation

Furosemide reversibly binds to the chloride-binding site of the apical Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle. By blocking electroneutral Na-K-2Cl entry, it dissipates the lumen-positive transepithelial potential, which secondarily abolishes paracellular calcium and magnesium reabsorption.

Question 6Renal structure and transport

A 52-year-old man with essential hypertension is treated with hydrochlorothiazide. In addition to mild blood pressure reduction, 24-hour urine collection shows a significant reduction in urinary calcium excretion. What is the primary tubular mechanism underlying this hypocalciuric effect?

  1. Stimulation of calcium precipitation by alkaline urine in the medullary collecting duct
  2. Enhanced passive and active calcium reabsorption in the proximal and distal convoluted tubules Correct answer.
  3. Direct pharmacological opening of basolateral calcium-sensing receptors (CaSR)
  4. Suppression of parathyroid hormone secretion leading to diminished bone turnover

Explanation

Thiazides inhibit the apical NCCT cotransporter in the distal convoluted tubule, lowering intracellular sodium and stimulating basolateral 3Na+/Ca2+ exchange, which accelerates apical TRPV5-mediated transcellular calcium entry. Furthermore, volume contraction induces compensatory proximal tubule sodium and passive paracellular calcium reabsorption.

Question 7Renal structure and transport

An 18-year-old patient presents with refractory hypertension, severe hypokalemia, and metabolic alkalosis. Laboratory evaluation reveals suppressed plasma renin activity and undetectable serum aldosterone. Genetic sequencing reveals a mutation in the SCNN1B gene causing deletion of the proline-rich PY motif. What is the molecular consequence of this defect?

  1. Constitutive hyperphosphorylation of the mineralocorticoid receptor in principal cells
  2. Accelerated lysosomal degradation of ENaC via enhanced binding of the ubiquitin ligase Nedd4-2
  3. Failure of Nedd4-2 binding leading to decreased internalization and prolonged membrane retention of ENaC Correct answer.
  4. Inhibition of the basolateral Na/K-ATPase leading to intracellular sodium accumulation

Explanation

Liddle syndrome is caused by gain-of-function mutations in the beta or gamma subunits of ENaC (SCNN1B/SCNN1G) deleting the cytoplasmic PY motif. The ubiquitin ligase Nedd4-2 normally binds the PY motif to ubiquitinate ENaC for endocytosis and degradation; loss of this motif results in accumulation of active ENaC channels at the apical membrane.

Question 8Renal structure and transport

A 34-year-old woman with primary Sjogren syndrome presents with recurrent nephrolithiasis, severe muscle weakness, hypokalemia, and hyperchloremic non-anion gap metabolic acidosis with a persistent urine pH of 6.8. A defect in which transport protein located on cortical collecting duct alpha-intercalated cells is responsible?

  1. Apical cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel
  2. Apical sodium-potassium-2-chloride cotransporter isoform B (NKCC2)
  3. Apical sodium-bicarbonate cotransporter NBCn1 (SLC4A7)
  4. Apical vacuolar H+-ATPase or basolateral anion exchanger 1 (AE1 / SLC4A1) Correct answer.

Explanation

Classical distal (Type 1) RTA is caused by dysfunction of alpha-intercalated cells in the collecting duct, specifically involving apical vacuolar H+-ATPase, H+/K+-ATPase, or basolateral anion exchanger 1 (AE1/SLC4A1). In auto-immune diseases like Sjogren syndrome, autoantibodies or lymphocytic infiltration disrupt these acid-secreting pumps.

Question 9Renal structure and transport

A 4-year-old child presents with heavy proteinuria, hypoalbuminemia, and generalized edema that fails to achieve remission after 8 weeks of daily oral prednisone. Renal biopsy demonstrates focal segmental glomerulosclerosis. Genetic analysis reveals a homozygous missense mutation in the NPHS2 gene. What is the normal physiological function of the protein product?

  1. Scaffolding and anchoring nephrin within cholesterol-rich lipid raft microdomains of the slit diaphragm Correct answer.
  2. Acting as the principal extracellular matrix ligand binding glomerular endothelial cells to the GBM
  3. Phosphorylating heavy-chain immunoglobulins inside the endoplasmic reticulum of mesangial cells
  4. Directly opening apical sodium channels in response to mechanical shear stress in podocyte foot processes

Explanation

NPHS2 encodes podocin, an integral hairpin membrane protein of the stomatin/prohibitin/flotillin family that localizes to lipid rafts at the insertion site of the slit diaphragm. Podocin oligomerizes to form scaffolding microdomains that recruit and stabilize nephrin and CD2AP at the podocyte plasma membrane.

Question 10Renal structure and transport

During experimental perfusion of the single nephron, an increase in sodium chloride delivery to the macula densa elicits a prompt decline in single-nephron glomerular filtration rate (SNGFR). Which signaling cascade mediates this tubuloglomerular feedback (TGF) response?

  1. Release of nitric oxide from macula densa causing selective afferent arteriolar dilation
  2. NaCl uptake via apical NKCC2 stimulating basolateral ATP/adenosine release to activate vascular A1 receptors Correct answer.
  3. PGE2 secretion from macula densa stimulating renin exocytosis from juxtaglomerular granular cells
  4. Direct entry of chloride into juxtaglomerular cells through CFTR activating endothelin-B receptors

Explanation

In tubuloglomerular feedback (TGF), elevated luminal NaCl concentration enhances uptake by apical NKCC2 cotransporters in macula densa cells. This increases basolateral ATP release, which is rapidly hydrolyzed to adenosine; adenosine acts on vascular A1 receptors on the afferent arteriole to elevate cytosolic calcium, causing afferent vasoconstriction and reducing GFR.

Question 11Renal structure and transport

A 45-year-old male with chronic metabolic acidosis secondary to chronic diarrhea undergoes evaluation of proximal tubular function. Under systemic acidemic conditions, which molecular adaptation occurs at the brush-border membrane of the proximal convoluted tubule to augment proton secretion?

  1. Internalization and lysosomal degradation of sodium-hydrogen exchanger 3 (NHE3)
  2. Downregulation of apical carbonic anhydrase IV transcription
  3. Transcriptional upregulation, exocytosis, and activation of apical sodium-hydrogen exchanger 3 (NHE3) Correct answer.
  4. Phosphorylation of basolateral Na/K-ATPase leading to complete cessation of sodium pumping

Explanation

Metabolic acidosis induces a rapid and sustained adaptive response in the proximal tubule characterized by exocytic insertion, reduced endocytic retrieval, and transcriptional upregulation of apical NHE3 (SLC9A3). This increases apical H+ extrusion into the tubular lumen to maximize bicarbonate titration and ammonium secretion.

Question 12Renal structure and transport

A 25-year-old athlete runs a marathon in a hot climate. In response to elevated circulating arginine vasopressin, maximal urinary concentration reaches 1200 mOsm/kg. Urea reabsorption into the inner medullary interstitium accounts for nearly half of this hypertonicity. Which specific transporters facilitate this urea recycling?

  1. Sodium-dependent urea cotransporters SGLT1 and SGLT2 in the medullary rays
  2. Basolateral anion exchangers AE1 and AE2 in the outer stripe of the outer medulla
  3. Aquaporin-1 water channels functioning as bidirectional urea uniporters in the descending limb
  4. Vasopressin-regulated UT-A1 and UT-A3 facilitated urea transporters in the inner medullary collecting duct Correct answer.

Explanation

Vasopressin activates V2 receptors on inner medullary collecting duct (IMCD) cells, stimulating cAMP-dependent phosphorylation and apical insertion of UT-A1 and basolateral UT-A3. This facilitates rapid passive exit of urea into the inner medullary interstitium, maintaining the corticomedullary osmotic gradient essential for water reabsorption.

Question 13Renal structure and transport

A 2-year-old child presents with failure to thrive, nephrocalcinosis, recurrent episodes of tetany, marked hypomagnesemia, and hypercalciuria with normal renal function. Genetic testing demonstrates a pathogenic mutation in the CLDN16 gene. What is the physiological mechanism responsible for this clinical presentation?

  1. Disruption of tight junction claudin complexes abolishing paracellular reabsorption of Mg2+ and Ca2+ in the thick ascending limb Correct answer.
  2. Defective transcellular active magnesium transport via apical TRPM6 channels in the distal convoluted tubule
  3. Constitutive activation of the basolateral calcium-sensing receptor (CaSR) suppressing parathyroid hormone release
  4. Impaired apical Na-K-2Cl cotransport resulting in severe hypokalemic metabolic alkalosis without nephrocalcinosis

Explanation

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is caused by autosomal recessive mutations in CLDN16 or CLDN19. These claudins form cation-selective paracellular tight junction pores in the thick ascending limb; loss of their function prevents passive voltage-driven paracellular reabsorption of magnesium and calcium.

Question 14Renal structure and transport

A 3-month-old infant is diagnosed with antenatal Bartter syndrome accompanied by bilateral sensorineural hearing loss. Genetic analysis reveals a homozygous loss-of-function mutation in the BSND gene. Which protein is encoded by this gene, and what is its specific physiological role?

  1. An accessory beta-subunit of the voltage-gated potassium channel ROMK in outer medullary collecting ducts
  2. Barttin, an indispensable accessory beta-subunit required for basolateral membrane insertion of CLC-Ka and CLC-Kb chloride channels Correct answer.
  3. A scaffolding protein that anchors NKCC2 cotransporters to the apical brush border of the proximal tubule
  4. A sensorineural cochlear transcription factor that downregulates endolymphatic calcium-sensing receptors

Explanation

Bartter syndrome Type 4 is caused by mutations in BSND, which encodes barttin. Barttin acts as an essential chaperone and beta-subunit for both CLC-Ka and CLC-Kb chloride channels in the basolateral membrane of the thick ascending limb and in the stria vascularis of the inner ear, explaining the combination of salt-wasting tubulopathy and sensorineural deafness.

Question 15Renal structure and transport

A 28-year-old woman is evaluated for fatigue, muscle cramps, and blood pressure of 100/60 mmHg. Serum potassium is 2.9 mEq/L, serum bicarbonate is 31 mEq/L, and serum magnesium is 1.2 mg/dL. Fractional excretion of chloride is elevated, but 24-hour urine calcium excretion is markedly reduced at 40 mg/day (calcium-to-creatinine ratio < 0.05). Which transporter mutation is the hallmark of this disorder?

  1. Inactivating mutation of the basolateral Na/H exchanger NHE1 in the cortical thick ascending limb
  2. Activating mutation of the apical calcium uniporter TRPV5 in the inner medullary collecting duct
  3. Inactivating mutation of the apical thiazide-sensitive Na-Cl cotransporter (NCCT) encoded by SLC12A3 Correct answer.
  4. Gain-of-function mutation of the basolateral chloride channel CLC-Kb in the loop of Henle

Explanation

Gitelman syndrome is an autosomal recessive disorder caused by inactivating mutations in SLC12A3, encoding the thiazide-sensitive NCCT in the distal convoluted tubule. The clinical phenotype includes hypokalemic metabolic alkalosis, hypomagnesemia, and profound hypocalciuria (differentiating it from Bartter syndrome, which presents with hypercalciuria).

Question 16Renal structure and transport

A patient with congenital nephrogenic diabetes insipidus is identified to harbor an X-linked loss-of-function mutation in the AVPR2 gene. Which intracellular downstream signaling event fails to occur in the medullary collecting duct principal cells?

  1. Gs protein coupling, adenylate cyclase activation, cyclic AMP elevation, and protein kinase A-mediated phosphorylation of Aquaporin-2 Correct answer.
  2. Gi protein inhibition, phospholipase C activation, IP3 generation, and calcium-induced exocytosis of Aquaporin-1
  3. Gq protein signaling, protein kinase C downregulation, and clathrin-dependent endocytosis of Aquaporin-3
  4. Direct tyrosine kinase phosphorylation of basolateral Aquaporin-4 without involvement of second messengers

Explanation

Arginine vasopressin binds to basolateral V2 receptors (AVPR2) coupled to Gs, which activates adenylate cyclase to produce cyclic AMP. cAMP activates protein kinase A (PKA), which phosphorylates Aquaporin-2 (AQP2) at Ser256, triggering exocytic trafficking of AQP2 storage vesicles to the apical membrane.

Question 17Renal structure and transport

A decrease in luminal sodium chloride delivery to the macula densa triggers a rapid stimulation of renin synthesis and exocytosis by juxtaglomerular (JG) cells. Which enzymatic and paracrine pathway within the juxtaglomerular apparatus couples this macula densa sensing to JG cell renin release?

  1. Inhibition of neuronal nitric oxide synthase (nNOS) decreasing cyclic GMP in juxtaglomerular cells
  2. Upregulation of cyclooxygenase-2 (COX-2) and microsomal PGE synthase-1, producing prostaglandin E2 that activates JG cell EP4 receptors Correct answer.
  3. Activation of membrane-bound endothelin-converting enzyme generating endothelin-1 to stimulate JG cell calcium influx
  4. Direct intercellular diffusion of calcium through open gap junctions from macula densa to suppress adenylate cyclase

Explanation

Low luminal NaCl delivery sensed by NKCC2 leads to intracellular chloride depletion in macula densa cells. This activates cyclooxygenase-2 (COX-2) and microsomal PGE synthase-1 (mPGES-1) to produce prostaglandin E2 (PGE2), which diffuses to neighboring JG cells and binds EP2/EP4 receptors, elevating cAMP and stimulating renin secretion.

Question 18Renal structure and transport

A 16-year-old male presents with microscopic hematuria, sensorineural deafness, and anterior lenticonus. Renal biopsy reveals longitudinal splitting, lamellation, and alternating thinning and thickening of the glomerular basement membrane ('basket-weave' appearance). Which molecular defect in type IV collagen underlies the primary pathogenesis of this condition?

  1. Defective assembly of the embryonic alpha-1/alpha-1/alpha-2 (IV) protomers into the mature tubular basement membrane
  2. Pathogenic autoantibodies targeting the non-collagenous 1 (NC1) domain of alpha-3 (IV) collagen
  3. Failure of developmental isotype switching from the embryonic alpha-1/alpha-1/alpha-2 network to the mature alpha-3/alpha-4/alpha-5 (IV) heterotrimer network Correct answer.
  4. Overproduction of alpha-5 (IV) chains leading to abnormal accumulation of heparan sulfate proteoglycans in the mesangium

Explanation

In normal glomerular maturation, embryonic alpha-1.alpha-1.alpha-2 (IV) collagen is replaced by the biochemically robust alpha-3.alpha-4.alpha-5 (IV) network. In X-linked Alport syndrome (COL4A5 mutations), this critical developmental isotype switch fails, leaving a fragile embryonic basement membrane susceptible to endopeptidase degradation, splitting, and progressive sclerosis.

Question 19Electrolytes and acid-base interpretation

A critically ill 64-year-old woman with septic shock is admitted to the intensive care unit. Serum chemistry shows: sodium 138 mEq/L, chloride 104 mEq/L, bicarbonate 12 mEq/L, and serum albumin 1.6 g/dL (normal: 4.0 g/dL). What is the patient's albumin-corrected serum anion gap, and what is the primary acid-base diagnosis?

  1. Corrected anion gap is 28 mEq/L, revealing an underlying high anion gap metabolic acidosis Correct answer.
  2. Corrected anion gap is 16 mEq/L, indicating a pure normal anion gap (hyperchloremic) metabolic acidosis
  3. Corrected anion gap is 22 mEq/L, indicating a mixed respiratory acidosis and metabolic alkalosis
  4. Corrected anion gap is 12 mEq/L, confirming that no unmeasured organic anions are present

Explanation

The observed anion gap is Na - (Cl + HCO3) = 138 - (104 + 12) = 22 mEq/L. Because albumin is the primary unmeasured circulating anion, the anion gap falls by 2.5 mEq/L for every 1.0 g/dL drop in serum albumin below 4.0 g/dL. The correction is: Observed AG + 2.5 * (4.0 - 1.6) = 22 + (2.5 * 2.4) = 22 + 6 = 28 mEq/L, unmasking severe high anion gap metabolic acidosis (e.g., lactic acidosis).

Question 20Electrolytes and acid-base interpretation

A 32-year-old man with type 1 diabetes presents with nausea, vomiting, and severe diarrhea. Laboratory tests reveal: sodium 140 mEq/L, chloride 115 mEq/L, bicarbonate 8 mEq/L, and normal serum albumin. The serum anion gap is 17 mEq/L (baseline: 11 mEq/L). Which acid-base disorder is identified by calculating the delta-delta ratio (Delta AG / Delta HCO3)?

  1. Isolated pure high anion gap metabolic acidosis without any accompanying disturbance
  2. Mixed high anion gap metabolic acidosis and concurrent normal anion gap (hyperchloremic) metabolic acidosis Correct answer.
  3. Mixed high anion gap metabolic acidosis and concurrent pre-existing metabolic alkalosis
  4. Triple acid-base disorder with profound respiratory alkalosis and normal anion gap acidosis

Explanation

Delta AG = Measured AG (17) - Normal AG (11) = 6 mEq/L. Delta HCO3 = Normal HCO3 (24) - Measured HCO3 (8) = 16 mEq/L. The delta-delta ratio is Delta AG / Delta HCO3 = 6 / 16 = 0.38. A ratio < 0.4 - 0.8 indicates that bicarbonate has fallen significantly more than the increase in unmeasured anions, confirming a mixed high anion gap acidosis (diabetic ketoacidosis) and normal anion gap acidosis (diarrheal bicarbonate loss).

Question 21Electrolytes and acid-base interpretation

A 54-year-old male with chronic alcohol use disorder presents with profound generalized weakness. Serum potassium is 2.4 mEq/L and serum magnesium is 1.1 mg/dL. Despite infusion of 120 mEq of intravenous potassium chloride over 24 hours, the repeat serum potassium remains 2.5 mEq/L. By which physiological mechanism does hypomagnesemia perpetuate this refractory renal potassium wasting?

  1. Direct toxic destruction of the basolateral Na/K-ATPase in principal cells
  2. Loss of intrinsic intracellular magnesium inhibition of apical ROMK channels leading to uninhibited potassium secretion Correct answer.
  3. Competitive displacement of potassium by magnesium at the luminal ENaC pore
  4. Profound inhibition of aldosterone synthesis in the adrenal zona glomerulosa

Explanation

Intracellular magnesium normally exerts a voltage-dependent pore-blocking effect on apical ROMK (KCNJ1) channels in the thick ascending limb and cortical collecting duct, preventing excessive potassium efflux into the lumen. Hypomagnesemia relieves this physiological blockade, resulting in uninhibited, rapid renal potassium secretion that is refractory to potassium repletion until magnesium is corrected.

Question 22Electrolytes and acid-base interpretation

A 48-year-old man with human immunodeficiency virus on long-term tenofovir disoproxil fumarate presents with bone pain, hypophosphatemia, normoglycemic glycosuria, generalized aminoaciduria, and hyperchloremic metabolic acidosis. Bicarbonate infusion is initiated to restore serum bicarbonate to normal (24 mEq/L). What fractional excretion of bicarbonate (FE-HCO3) is diagnostic of proximal (Type 2) RTA under these conditions?

  1. Fractional excretion of bicarbonate strictly less than 1%
  2. Fractional excretion of bicarbonate between 2% and 4%
  3. Fractional excretion of bicarbonate remaining persistently at zero
  4. Fractional excretion of bicarbonate markedly elevated above 15% to 20% Correct answer.

Explanation

In proximal (Type 2) RTA, the proximal tubule's threshold for bicarbonate reabsorption is lowered (typically 14-18 mEq/L). When intravenous bicarbonate is infused to raise serum bicarbonate to normal (> 22-24 mEq/L), the diseased proximal tubule is flooded and unable to reabsorb the excess, causing massive bicarbonaturia with a fractional excretion of bicarbonate > 15-20%.

Question 23Electrolytes and acid-base interpretation

A 62-year-old male with long-standing type 2 diabetes mellitus and CKD stage 3b (eGFR 38 mL/min/1.73 m2) presents with persistent hyperkalemia (potassium 5.6-5.9 mEq/L) and hyperchloremic normal anion gap metabolic acidosis. Urine pH is 5.0. Plasma renin activity and serum aldosterone are both inappropriately low. Which pathophysiology defines this syndrome of hyporeninemic hypoaldosteronism (Type 4 RTA)?

  1. Hyperkalemia suppresses renal ammoniagenesis in the proximal tubule, reducing buffer availability for proton elimination Correct answer.
  2. Direct destruction of collecting duct intercalated cells by circulating advanced glycation end-products
  3. Hyperactive pendrin exchangers causing excessive secretion of bicarbonate into the urine
  4. Severe deficiency of carbonic anhydrase II causing secondary generalized Fanconi syndrome

Explanation

Type 4 RTA is characterized by hypoaldosteronism or aldosterone resistance. Aldosterone deficiency decreases ENaC-mediated sodium uptake, dissipating the lumen-negative potential and impairing potassium and proton secretion. Importantly, the resulting hyperkalemia directly inhibits proximal tubular synthesis and transfer of ammonium (NH4+), reducing the urinary buffer needed to excrete acid, although the collecting duct can still lower urine pH < 5.3.

Question 24Electrolytes and acid-base interpretation

A 23-year-old male is evaluated for glue-sniffing (toluene toxicity) presenting with severe hyperchloremic metabolic acidosis and hypokalemia. Serum anion gap is 12 mEq/L. Urine electrolytes show: Na 60 mEq/L, K 45 mEq/L, and Cl 50 mEq/L (Urine net charge = +55 mEq/L). Measured urine osmolality is 580 mOsm/kg, urine urea nitrogen is 280 mg/dL, and urine glucose is negative. What does the calculated urine osmolal gap reveal in this patient?

  1. Urine osmolal gap is 50 mOsm/kg, confirming impaired renal ammonium excretion due to classical distal RTA
  2. Urine osmolal gap is 270 mOsm/kg, confirming robust excretion of ammonium alongside unmeasured hippurate anions Correct answer.
  3. Urine osmolal gap is zero, indicating that total solute excretion consists entirely of sodium and potassium salts
  4. Urine osmolal gap is negative, demonstrating severe medullary washout and complete loss of concentrating ability

Explanation

Calculated urine osmolality = 2*(UNa + UK) + UUN/2.8 + Glucose/18 = 2*(60 + 45) + 280/2.8 + 0 = 210 + 100 = 310 mOsm/kg. The urine osmolal gap (UOG) = Measured (580) - Calculated (310) = 270 mOsm/kg. Approximately half of this gap represents NH4+ (about 135 mEq/L). In toluene abuse, massive hippurate excretion forces ammonium excretion with hippurate rather than chloride, making the urine net charge falsely positive while the urine osmolal gap correctly reveals robust ammonium production.

Question 25Electrolytes and acid-base interpretation

A 35-year-old female presents with severe metabolic alkalosis (pH 7.54, bicarbonate 38 mEq/L) and hypokalemia (potassium 2.8 mEq/L). Her blood pressure is 110/70 mmHg without edema. Urine electrolytes demonstrate a spot urine chloride concentration of 8 mEq/L. Which clinical etiology is most consistent with this presentation?

  1. Unilateral aldosterone-producing adrenal adenoma (Conn syndrome)
  2. Persistent ectopic adrenocorticotropic hormone (ACTH) production by small cell carcinoma
  3. Persistent upper gastrointestinal loss of hydrochloric acid due to recurrent vomiting or bulimia Correct answer.
  4. Gitelman syndrome secondary to homozygous SLC12A3 inactivating mutations

Explanation

Metabolic alkalosis is classified by urinary chloride into chloride-responsive (urine Cl < 20 mEq/L) and chloride-resistant (urine Cl > 20 mEq/L). In upper GI loss (vomiting, nasogastric suction), loss of HCl generates alkalosis, while volume depletion and chloride depletion maintain it; the kidney avidly conserves chloride (urine Cl < 10-20 mEq/L), and the alkalosis corrects with 0.9% normal saline infusion.

Question 26Electrolytes and acid-base interpretation

A 26-year-old female runner collapses at the end of a marathon with status epilepticus and non-reactive pupils due to exercise-associated hyponatremic encephalopathy. Immediate point-of-care serum sodium is 114 mEq/L. What is the immediate, life-saving therapeutic intervention recommended by international consensus guidelines?

  1. A rapid IV bolus of 100 to 150 mL of 3% hypertonic saline over 10-20 minutes, repeatable twice if seizures persist Correct answer.
  2. A slow continuous infusion of 0.9% normal saline at 100 mL/hour over 24 hours
  3. Administration of oral tolvaptan 15 mg tablet crushed through a nasogastric tube
  4. Infusion of 5% dextrose in water at 200 mL/hour to prevent cerebral dehydration

Explanation

In patients presenting with severe, life-threatening neurological manifestations of hyponatremia (seizures, coma, brain herniation), the immediate goal is a rapid 4-6 mEq/L elevation in serum sodium to reverse cerebral edema and reduce intracranial pressure. Consensus guidelines mandate a prompt IV bolus of 100-150 mL of 3% hypertonic saline over 10-20 minutes, repeated up to two additional times if symptoms persist.

Question 27Electrolytes and acid-base interpretation

A 55-year-old woman with severe thiazide-induced hyponatremia (sodium 108 mEq/L) is admitted to the ICU. After stopping the thiazide and initiating 0.9% saline, the patient abruptly begins voiding 800 mL/hour of dilute urine (specific gravity < 1.003), and serum sodium jumps to 118 mEq/L in 6 hours. Which pharmacological rescue strategy ('DDAVP clamp') is indicated to arrest this rapid overcorrection?

  1. Switching intravenous fluids to 3% hypertonic saline at a rate of 50 mL/hour
  2. Administering intravenous 1-2 mcg of desmopressin (DDAVP) combined with 5% dextrose in water (D5W) infusion Correct answer.
  3. Administering intravenous furosemide 40 mg to accelerate free water clearance
  4. Starting oral urea sachets (30 grams twice daily) to promote osmotic diuresis

Explanation

When the cause of hyponatremia is reversed (e.g., discontinuation of thiazides, volume resuscitation in prerenal states), endogenous vasopressin drops precipitously, triggering an abrupt water diuresis that can cause rapid overcorrection. The 'DDAVP clamp' strategy utilizes parenteral desmopressin (1-2 mcg IV/SC every 6-8 hours) to arrest free water diuresis combined with D5W infusion to re-lower or stabilize serum sodium within safe targets.

Question 28Electrolytes and acid-base interpretation

An 80-year-old demented man residing in a nursing home presents with fever, dry mucous membranes, confusion, and serum sodium of 168 mEq/L. His current weight is 60 kg. Assuming a total body water fraction of 0.50, what is his approximate free water deficit, and what is the maximum safe rate of serum sodium reduction?

  1. Free water deficit is approximately 2.0 liters; reduce sodium by 24 mEq/L in 12 hours
  2. Free water deficit is approximately 12.0 liters; reduce sodium by 20 mEq/L in 24 hours
  3. Free water deficit is approximately 8.5 liters; reduce sodium by no more than 15 mEq/L in 6 hours
  4. Free water deficit is approximately 6.0 liters; reduce sodium by no more than 10 to 12 mEq/L in 24 hours Correct answer.

Explanation

Free water deficit = TBW * [(Current Na / 140) - 1]. For an elderly male, TBW factor is 0.50: TBW = 60 * 0.50 = 30 L. Deficit = 30 * [(168 / 140) - 1] = 30 * [1.2 - 1] = 30 * 0.2 = 6.0 Liters. In chronic hypernatremia (> 48 hours), brain cells accumulate idiogenic osmoles; lowering sodium faster than 10-12 mEq/L per 24 hours (0.5 mEq/L/hr) can drive water into cerebral cells, precipitating fatal cerebral edema and seizures.

Question 29Electrolytes and acid-base interpretation

A 56-year-old malnourished female with chronic osteoarthritic knee pain treated with 4 grams of oral acetaminophen daily for 6 months presents with tachypnea, confusion, and profound high anion gap metabolic acidosis (pH 7.15, bicarbonate 7 mEq/L, anion gap 26 mEq/L). Serum lactate, ketoacids, salicylate, methanol, and ethylene glycol are undetectable. Which organic acid accumulation explains this metabolic disorder?

  1. Pyroglutamic acid (5-oxoproline) accumulation resulting from chronic glutathione depletion Correct answer.
  2. Hippuric acid accumulation resulting from industrial glycol ether inhalation
  3. D-lactic acid accumulation from intestinal bacterial fermentation of starch
  4. Beta-aminoisobutyric acid accumulation from hypercatabolic thymine degradation

Explanation

Chronic acetaminophen administration in malnourished, septic, or cirrhotic patients depletes hepatic and renal intracellular glutathione. Glutathione normally exerts feedback inhibition on gamma-glutamylcysteine synthetase in the gamma-glutamyl cycle. Loss of this inhibition diverts gamma-glutamylcysteine into 5-oxoproline (pyroglutamic acid), causing severe high anion gap metabolic acidosis (5-oxoprolinuria).

Question 30Electrolytes and acid-base interpretation

A 38-year-old male with Crohn disease who previously underwent extensive small bowel resection (remaining small bowel 60 cm) presents with episodic confusion, slurred speech, ataxia, and aggressive behavior occurring 2 hours after high-carbohydrate meals. Blood gas reveals high anion gap metabolic acidosis. A standard automated enzymatic clinical laboratory assay reports a normal serum lactate level (1.0 mmol/L). What is the biochemical explanation for this clinical discrepancy?

  1. The acidosis is driven entirely by unmeasured acetoacetate that co-elutes with physiological pyruvate
  2. Colonic bacterial overgrowth of Lactobacillus metabolizes unabsorbed carbohydrates into D-lactate, which is undetectable by standard L-lactate dehydrogenase assays Correct answer.
  3. Massive cellular release of inorganic pyrophosphate mimicking true organic acidemia
  4. Complete inhibition of pyruvate dehydrogenase by thiamine pyrophosphate deficiency

Explanation

In short bowel syndrome, unabsorbed carbohydrates enter the colon and are fermented by anaerobic bacteria (e.g., Lactobacillus species) into D-lactic acid. D-lactic acid is absorbed into the systemic circulation and crosses the blood-brain barrier, causing neuro-glycopenic symptoms. Standard clinical laboratory lactate assays utilize stereospecific L-lactate dehydrogenase and cannot detect the D-isomer.

Question 31Electrolytes and acid-base interpretation

A 45-year-old critically ill trauma patient in the ICU develops refractory delirium tremens managed with a continuous intravenous lorazepam infusion (15 mg/hour) for 5 days. Laboratory evaluation demonstrates a widening osmolar gap (28 mOsm/kg) and an escalating high anion gap metabolic acidosis with acute kidney injury. Which solvent vehicle present in parenteral lorazepam is responsible for this toxicity?

  1. Diethylene glycol, which is metabolized into glycolic and oxalic acids
  2. Polyethylene glycol 400, which causes direct non-anion gap tubular obstruction
  3. Propylene glycol, which is metabolized by alcohol dehydrogenase into L- and D-lactic acid Correct answer.
  4. Benzyl alcohol, which causes severe respiratory gasping syndrome without metabolic acidosis

Explanation

Propylene glycol is a solvent vehicle utilized in parenteral formulations of lorazepam, diazepam, and trimethoprim-sulfamethoxazole. High-dose continuous infusions lead to propylene glycol accumulation, producing an elevated serum osmolar gap. It is metabolized by alcohol dehydrogenase into D- and L-lactic acid and pyruvic acid, generating severe high anion gap metabolic acidosis and proximal tubular injury.

Question 32Glomerular and systemic renal disease

A 5-year-old boy presents with abrupt onset periorbital edema, scrotal swelling, ascites, and 4+ proteinuria on dipstick. Urine microscopy shows oval fat bodies with no erythrocytes. 24-hour urine protein is 5.2 g, serum albumin is 1.8 g/dL, and serum complement levels (C3 and C4) are normal. What are the characteristic findings on diagnostic renal biopsy?

  1. Normal glomeruli on light microscopy, negative immunofluorescence, and diffuse podocyte foot process effacement on electron microscopy Correct answer.
  2. Segmental glomerular sclerosis with hyalinosis on light microscopy and granular IgM in sclerotic foci
  3. Diffuse glomerular basement membrane thickening with 'spike and dome' pattern on silver stain
  4. Mesangial hypercellularity with bright mesangial IgA and C3 deposition on immunofluorescence

Explanation

Minimal change disease (MCD) is the most common cause of nephrotic syndrome in children. Light microscopy appears completely normal (or shows only mild lipid accumulation in tubules), immunofluorescence is negative for immune deposits, and electron microscopy reveals diffuse, extensive effacement of visceral epithelial cell (podocyte) foot processes.

Question 33Glomerular and systemic renal disease

A 52-year-old previously healthy male presents with heavy proteinuria (8.5 g/24 hours), bilateral lower extremity pitting edema, and hypoalbuminemia (2.1 g/dL). Serum creatinine is 1.0 mg/dL. A circulating autoantibody test is sent. Which autoantibody is directed against the primary target podocyte antigen in 70% to 80% of adult primary membranous nephropathy?

  1. Anti-neutrophil cytoplasmic antibodies directed against proteinase-3 (anti-PR3)
  2. Anti-double stranded DNA (anti-dsDNA) antibodies with hypocomplementemia
  3. Autoantibodies targeting the M-type phospholipase A2 receptor (anti-PLA2R) Correct answer.
  4. Autoantibodies targeting the non-collagenous domain of alpha-3 type IV collagen

Explanation

Autoantibodies against the M-type phospholipase A2 receptor (anti-PLA2R), an antigen expressed on human podocyte membranes, are present in 70% to 80% of patients with primary membranous nephropathy. Anti-PLA2R titers correlate directly with disease activity, proteinuria severity, response to immunosuppressive therapy, and post-transplant recurrence risk.

Question 34Glomerular and systemic renal disease

A 22-year-old college student presents to the clinic with visible gross hematuria (coca-cola colored urine). He notes that he developed a severe sore throat, fever, and pharyngitis only 24 hours prior. His blood pressure is 125/80 mmHg and serum creatinine is 0.9 mg/dL. What is the most likely diagnosis?

  1. Post-streptococcal acute glomerulonephritis (PSGN)
  2. Granulomatosis with polyangiitis (GPA)
  3. Alport syndrome with acute interstitial nephritis
  4. Primary IgA nephropathy (Berger disease) Correct answer.

Explanation

Primary IgA nephropathy classically manifests in young adults as episodic macroscopic hematuria that occurs concurrently with or within 24 to 48 hours of an upper respiratory or gastrointestinal infection (synpharyngitic hematuria). In contrast, post-streptococcal glomerulonephritis exhibits a characteristic latent period of 1 to 3 weeks following pharyngitis.

Question 35Glomerular and systemic renal disease

A 26-year-old woman with systemic lupus erythematosus presents with malar rash, arthralgias, new hypertension (155/95 mmHg), proteinuria of 3.2 g/24h, and active urinary sediment with RBC casts. Serum C3 and C4 are markedly depressed and anti-dsDNA titer is high. Renal biopsy reveals endocapillary proliferation in 65% of glomeruli, subendothelial immune deposits, and 'wire-loop' lesions. According to the ISN/RPS classification, which class of lupus nephritis is this?

  1. Class IV: Diffuse proliferative lupus nephritis (affecting >= 50% of glomeruli) Correct answer.
  2. Class II: Mesangial proliferative lupus nephritis
  3. Class III: Focal proliferative lupus nephritis (affecting < 50% of glomeruli)
  4. Class V: Pure membranous lupus nephritis

Explanation

Diffuse proliferative lupus nephritis involves at least half of sampled glomeruli in the stated classification. The described 65 percent involvement therefore supports class IV; activity, chronicity and the complete clinical context also inform treatment.

Question 36Glomerular and systemic renal disease

A 30-year-old male with biopsy-proven IgA nephropathy undergoes pathological scoring using the revised Oxford Classification (MEST-C score). Which component of the MEST-C score is specifically defined as extracapillary cellular or fibrocellular proliferation involving >= 10% of the glomerular circumference, and independently predicts rate of renal function decline?

  1. Mesangial hypercellularity (M score, M0 vs M1)
  2. Segmental glomerulosclerosis (S score, S0 vs S1)
  3. Crescent formation (C score: C0 = 0%, C1 = 1-24%, C2 >= 25% of glomeruli) Correct answer.
  4. Tubular atrophy and interstitial fibrosis (T score, T0 vs T1 vs T2)

Explanation

The revised Oxford Classification for IgA nephropathy includes the MEST-C score: M (mesangial hypercellularity), E (endocapillary hypercellularity), S (segmental glomerulosclerosis), T (tubular atrophy/interstitial fibrosis), and C (crescents). C1 indicates cellular/fibrocellular crescents in > 0% but < 25% of glomeruli, while C2 indicates crescents in >= 25% of glomeruli, identifying patients at high risk of rapid progression who may benefit from immunosuppressive therapy.

Question 37Glomerular and systemic renal disease

A 24-year-old female presents with biopsy-proven active Class IV-G (A) diffuse proliferative lupus nephritis with heavy proteinuria (4.2 g/24h) and active urinary sediment. Serum creatinine is 1.4 mg/dL. According to the KDIGO and ACR guidelines, which regimen represents an established, evidence-based initial induction therapy?

  1. Intravenous pulse methylprednisolone followed by oral glucocorticoids combined with either Mycophenolate Mofetil (target 2-3 g/day) or low-dose IV Cyclophosphamide (Euro-Lupus regimen) Correct answer.
  2. High-dose intravenous immunoglobulin (IVIG) monotherapy monthly for 6 months
  3. Oral hydroxychloroquine monotherapy at 400 mg daily without systemic corticosteroids
  4. Oral azathioprine at 2 mg/kg/day combined with oral methotrexate

Explanation

Standard induction therapy for active Class III or IV lupus nephritis consists of pulse intravenous methylprednisolone followed by oral corticosteroids, combined with either mycophenolate mofetil (MMF, 2-3 g/day) or low-dose intravenous cyclophosphamide (the Euro-Lupus regimen: 500 mg IV every 2 weeks for 6 doses). Hydroxychloroquine is maintained as background therapy in all patients.

Question 38Glomerular and systemic renal disease

A 66-year-old male presents with rapidly progressive glomerulonephritis, serum creatinine 4.2 mg/dL, microscopic hematuria with dysmorphic red blood cells, and hemoptysis. Indirect immunofluorescence reveals a cytoplasmic ANCA (c-ANCA) pattern, and ELISA confirms high titers of anti-proteinase 3 (anti-PR3) antibodies. Renal biopsy demonstrates pauci-immune necrotizing and crescentic glomerulonephritis. What does the term 'pauci-immune' specifically signify on immunofluorescence microscopy?

  1. Dense, confluent, ribbon-like linear IgG staining along 100% of the capillary loops
  2. Scant or absent immunoglobulin and complement deposition within glomerular capillary loops and mesangium Correct answer.
  3. Heavy polyclonal 'full house' staining for IgG, IgA, IgM, C3, and C1q
  4. Bright, isolated mesangial IgA and C3 deposition without any IgG

Explanation

In ANCA-associated glomerulonephritis (AAV), the hallmark pathological term 'pauci-immune' designates the near or complete absence of significant immunoglobulin or complement deposition on immunofluorescence microscopy (0 to trace staining), distinguishing it from immune complex-mediated glomerulonephritis (e.g., lupus, IgAN) and anti-GBM disease (linear IgG).

Question 39Glomerular and systemic renal disease

A 21-year-old male smoker presents with acute oliguric renal failure, hemoptysis, and bilateral alveolar infiltrates. Renal biopsy demonstrates circumferential cellular crescents in 90% of glomeruli and intense, smooth, continuous linear IgG staining along the glomerular capillary walls on immunofluorescence. What is the specific molecular target of the pathogenic autoantibodies in this disorder?

  1. The extracellular domain of neutral endopeptidase on the podocyte foot process
  2. The heavy chain Fc fragment of secretory immunoglobulin A1 in the mesangium
  3. The non-collagenous domain of the alpha-5 chain of type IV collagen in Bowman capsule
  4. The non-collagenous 1 (NC1) domain of the alpha-3 chain of type IV collagen [alpha-3(IV)NC1] Correct answer.

Explanation

Anti-glomerular basement membrane (anti-GBM) disease (Goodpasture syndrome when pulmonary hemorrhage is present) is caused by autoantibodies directed against the non-collagenous 1 (NC1) domain of the alpha-3 chain of type IV collagen. The pathognomonic finding on immunofluorescence is bright, continuous linear IgG deposition along the GBM.

Question 40Glomerular and systemic renal disease

A 14-year-old girl is evaluated for partial acquired lipodystrophy (loss of subcutaneous adipose tissue from the face and upper torso), bilateral ocular fundus drusen, and hematuria. Renal biopsy shows dense, discontinuous, highly osmophilic ribbon-like deposits transforming the lamina densa of the glomerular basement membrane on electron microscopy. What is the definitive clinicopathological diagnosis?

  1. Collapsing focal segmental glomerulosclerosis associated with APOL1 risk alleles
  2. Dense deposit disease (DDD, formerly Type II membranoproliferative glomerulonephritis) Correct answer.
  3. Fibrillary glomerulonephritis with Congo-red negative DNAJB9-positive microfibrils
  4. Alport syndrome with juvenile diffuse leiomyomatosis of the esophagus

Explanation

Dense Deposit Disease (DDD) is a subtype of C3 glomerulopathy characterized pathognomonically on electron microscopy by extremely dark, ribbon-like, osmophilic intramembranous electron-dense deposits that replace and expand the lamina densa of the GBM. DDD is strongly associated with circulating C3NeF, acquired partial lipodystrophy (Barraquer-Simons syndrome), and retinal drusen.

Question 41Glomerular and systemic renal disease

A 28-year-old female presents with confusion, fever, petechiae, severe microangiopathic hemolytic anemia (schistocytes on blood smear), profound thrombocytopenia (platelet count 14,000/mcL), and mild renal insufficiency (creatinine 1.5 mg/dL). Coagulation profile (PT, PTT, fibrinogen) is completely normal. Which laboratory investigation is paramount to differentiate thrombotic thrombocytopenic purpura (TTP) from atypical hemolytic uremic syndrome (aHUS)?

  1. Stool polymerase chain reaction for Shiga-toxin genes stx1 and stx2
  2. Serum antibody testing for anti-factor H autoantibodies
  3. Plasma ADAMTS13 activity level, where severe deficiency (< 10%) confirms TTP Correct answer.
  4. Serum haptoglobin concentration and direct antiglobulin (Coombs) testing

Explanation

Severely reduced ADAMTS13 activity, conventionally below 10 percent, strongly supports thrombotic thrombocytopenic purpura in the appropriate thrombotic microangiopathy presentation. When clinical suspicion is high, urgent treatment should not be delayed while waiting for this test.

Question 42Acute injury, chronic disease and hypertension

A 68-year-old male develops acute kidney injury following an open abdominal aortic aneurysm repair. His baseline serum creatinine was 1.0 mg/dL. On postoperative day 2, his serum creatinine reaches 3.2 mg/dL, and his urine output over the preceding 14 hours has totaled 40 mL (less than 0.1 mL/kg/hour). According to KDIGO criteria, what stage of acute kidney injury does this represent?

  1. Stage 3 Acute Kidney Injury (SCr >= 3.0 times baseline and anuria/oliguria criteria met) Correct answer.
  2. Stage 1 Acute Kidney Injury (based solely on the short duration of oliguria)
  3. Stage 2 Acute Kidney Injury (SCr increased 2.0 to 2.9 times baseline)
  4. Stage 0 Subclinical Kidney Injury (requiring serum cystatin-C confirmation)

Explanation

A rise from a baseline creatinine of 1.0 to 3.2 mg/dL meets the threefold creatinine criterion for stage 3 acute kidney injury. The stated 14 hours of oliguria does not by itself establish the separate 24-hour severe-oliguria criterion.

Question 43Acute injury, chronic disease and hypertension

A 55-year-old male with septic shock develops severe oliguria and a rising serum creatinine. Freshly voided urine is examined under brightfield and phase-contrast microscopy. Which microscopic sediment finding provides the highest positive predictive value for the diagnosis of acute tubular necrosis (ATN)?

  1. Hyaline casts and occasional amorphous urate crystals
  2. Dysmorphic red blood cells with acanthocytes and erythrocyte casts
  3. Coarse muddy brown granular casts and renal tubular epithelial cell casts Correct answer.
  4. White blood cell casts accompanied by abundant calcium oxalate envelope crystals

Explanation

The presence of muddy brown granular casts (composed of sloughed tubular epithelial cell debris, Tamm-Horsfall mucoprotein, and denatured cytoplasmic organelles) alongside renal tubular epithelial cell (RTEC) casts is the hallmark urinary microscopic finding of acute tubular necrosis (ATN), carrying a positive predictive value exceeding 95% in clinical AKI scoring systems.

Question 44Acute injury, chronic disease and hypertension

A 64-year-old woman is treated with omeprazole for gastroesophageal reflux and ciprofloxacin for a complicated urinary tract infection. Ten days later, she develops low-grade fever, a maculopapular rash on her trunk, and serum creatinine rises from 0.8 to 2.6 mg/dL. Urinalysis reveals mild non-nephrotic proteinuria, sterile pyuria, and white blood cell casts. What is the most likely diagnosis?

  1. Acute cortical necrosis secondary to sepsis
  2. Renal atheroembolic microcrystalline disease
  3. Acute tubular necrosis induced by aminoglycoside toxicity
  4. Drug-induced acute interstitial nephritis (AIN) Correct answer.

Explanation

Drug-induced acute interstitial nephritis (AIN) is a cell-mediated hypersensitivity reaction occurring 7 to 14 days following exposure to common culprit drugs including proton pump inhibitors (omeprazole), fluoroquinolones (ciprofloxacin), beta-lactams, and NSAIDs. Classic findings include sterile pyuria, white blood cell casts, mild proteinuria, and occasionally fever and rash.

Question 45Acute injury, chronic disease and hypertension

A 62-year-old male with ischemic cardiomyopathy and an ejection fraction of 25% is admitted with acute pulmonary edema. Within 24 hours of presentation, his serum creatinine rises from 1.2 to 2.6 mg/dL despite maintaining a mean arterial pressure of 85 mmHg. Central venous pressure is markedly elevated at 22 cmH2O. According to the Ronco consensus classification, what type of cardiorenal syndrome is this, and what is the primary hemodynamic driver of the renal dysfunction?

  1. Type 3 Cardiorenal Syndrome driven by acute primary parenchymal renal ischemia
  2. Type 4 Cardiorenal Syndrome driven by chronic myocardial remodeling from uremic toxins
  3. Type 1 Cardiorenal Syndrome driven primarily by elevated central venous pressure and renal venous congestion Correct answer.
  4. Type 5 Cardiorenal Syndrome driven by acute systemic bacterial sepsis

Explanation

Type 1 Cardiorenal Syndrome is characterized by acute worsening of heart failure causing acute kidney injury. Hemodynamic investigations have established that elevated central venous pressure and subsequent renal venous congestion ('backward failure') is the primary physiological driver of reduced glomerular perfusion gradient [P_perf = MAP - CVP], often having a greater impact than reduced cardiac output ('forward failure').

Question 46Acute injury, chronic disease and hypertension

A 24-year-old marathon runner collapses in warm weather and is found unconscious after an unknown duration of immobilization. Laboratory results show: serum creatine kinase (CK) 85,000 U/L, potassium 6.2 mEq/L, and serum creatinine 3.8 mg/dL. Urinalysis dipstick is strongly positive for 'blood' (4+), but microscopic examination reveals only 0-1 intact red blood cells per high-power field. What is the fundamental mechanism of nephrotoxicity in this disorder?

  1. Intratubular myoglobin cast precipitation with Tamm-Horsfall protein, renal vasoconstriction, and direct heme-mediated tubular oxidative injury Correct answer.
  2. Massive excretion of free hemoglobin binding to megalin-cubilin complexes causing proximal tubular apoptosis
  3. Direct immunological complement attack against podocytes triggered by circulating troponin complexes
  4. Acute crystalline nephropathy caused by intrarenal precipitation of calcium carbonate monohydrate

Explanation

Rhabdomyolysis causes AKI through three synergistic mechanisms: (1) renal vasoconstriction induced by volume depletion and nitric oxide scavenging by heme, (2) direct cytotoxic injury to proximal tubular cells via free iron-driven Fenton chemistry producing reactive oxygen species, and (3) precipitation of myoglobin with Tamm-Horsfall protein in acidic distal tubular lumens forming obstructive casts. The dipstick detects the pseudoperoxidase activity of myoglobin heme, explaining 'blood' positivity with absent intact RBCs.

Question 47Acute injury, chronic disease and hypertension

In the early stages of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD), progressive loss of functional nephrons leads to phosphate retention. Which circulating phosphaturic hormone, secreted by osteocytes, rises earliest in the course of CKD to maintain normal serum phosphorus, and what is its effect on vitamin D metabolism?

  1. Parathyroid hormone, which stimulates renal 24-hydroxylase and increases 1,25-dihydroxyvitamin D synthesis
  2. Fibroblast Growth Factor 23 (FGF-23), which downregulates NaPi-IIa/c cotransporters and directly suppresses 1-alpha-hydroxylase (CYP27B1) Correct answer.
  3. Osteocalcin, which upregulates intestinal phosphate cotransporter NaPi-IIb and stimulates calcium release
  4. Klotho, which acts as a circulating monomer to degrade calcium-sensing receptors in parathyroid glands

Explanation

Fibroblast Growth Factor 23 (FGF-23) is secreted by osteocytes and osteoblasts in response to early phosphate accumulation. In the proximal tubule, FGF-23 binds the FGFR1-Klotho coreceptor complex, inducing endocytosis of apical sodium-phosphate cotransporters (NaPi-IIa and NaPi-IIc) to promote phosphaturia. Concurrently, FGF-23 strongly inhibits 1-alpha-hydroxylase (CYP27B1) and upregulates 24-hydroxylase (CYP24A1), causing a precipitous early decline in circulating 1,25-(OH)2D3 (calcitriol) that secondarily triggers hyperparathyroidism.

Question 48Acute injury, chronic disease and hypertension

A 54-year-old male with type 2 diabetes mellitus, hypertension, and stage 3a CKD with persistent albuminuria (UACR 450 mg/g) is already taking maximum-tolerated lisinopril and empagliflozin. Serum potassium is 4.3 mEq/L and eGFR is 52 mL/min/1.73 m2. Based on the FIDELIO-DKD and FIGARO-DKD trials, which novel therapeutic agent should be added to further slow CKD progression and reduce cardiovascular risk?

  1. Spironolactone, a steroidal mineralocorticoid receptor antagonist with high affinity for androgen receptors
  2. Aliskiren, a direct renin inhibitor acting synergistically with ACE inhibitors
  3. Finerenone, a nonsteroidal selective mineralocorticoid receptor antagonist that blocks inflammation and fibrosis Correct answer.
  4. Amlodipine, a dihydropyridine calcium channel blocker targeting postglomerular resistance

Explanation

The FIDELIO-DKD and FIGARO-DKD trials proved that finerenone, a selective nonsteroidal mineralocorticoid receptor antagonist (ns-MRA), significantly reduces the composite risk of CKD progression, kidney failure, cardiovascular death, and heart failure hospitalization in patients with CKD and type 2 diabetes already treated with optimized RAAS inhibitors. Unlike steroidal MRAs (spironolactone), finerenone binds MR with equal affinity, has a short half-life, lacks anti-androgenic side effects (no gynecomastia), and carries a lower incidence of severe hyperkalemia.

Question 49Acute injury, chronic disease and hypertension

A 28-year-old woman presents with severe refractory hypertension (blood pressure 185/110 mmHg). On physical examination, an epigastric systolic-diastolic abdominal bruit is auscultated. Renal CT angiography reveals alternating areas of stenosis and ectasia ('string-of-beads' appearance) located primarily in the middle and distal segments of the right main renal artery. What is the diagnosis, and what is the revascularization procedure of choice?

  1. Atherosclerotic renal artery stenosis; immediate surgical aortorenal bypass grafting
  2. Takayasu arteritis; high-dose pulse intravenous cyclophosphamide monotherapy
  3. Polyarteritis nodosa; long-term therapeutic anticoagulation with warfarin
  4. Fibromuscular dysplasia (medial fibroplasia); percutaneous transluminal renal angioplasty without stenting Correct answer.

Explanation

Fibromuscular dysplasia (FMD) is a non-atherosclerotic, non-inflammatory vascular disease affecting predominantly women aged 20-50. The medial fibroplasia subtype accounts for 80-90% of cases, producing the classic 'string-of-beads' sign in the mid-to-distal renal artery. The therapeutic procedure of choice for symptomatic renovascular hypertension is percutaneous transluminal angioplasty (PTRA) without primary stent placement, reserving stents only for procedural flow-limiting dissection.

Question 50Acute injury, chronic disease and hypertension

A 42-year-old male is evaluated for stage 3 resistant hypertension and persistent unprovoked hypokalemia (potassium 2.9 mEq/L). Screening reveals a plasma aldosterone concentration of 34 ng/dL and plasma renin activity of 0.2 ng/mL/h (aldosterone-to-renin ratio ARR = 170). Adrenal protocol CT demonstrates a 1.2 cm left adrenal nodule and normal right adrenal gland. What essential diagnostic step must be completed before referring this patient for curative laparoscopic unilateral adrenalectomy?

  1. Bilateral adrenal venous sampling (AVS) to differentiate unilateral aldosteronoma from bilateral adrenal hyperplasia Correct answer.
  2. Immediate left laparoscopic adrenalectomy based strictly on high-resolution thin-cut CT imaging findings
  3. Urine 24-hour collection for vanillylmandelic acid and fractionated catecholamines
  4. Low-dose dexamethasone suppression test combined with adrenocorticotropic hormone (ACTH) stimulation

Explanation

In primary aldosteronism, CT imaging is notoriously inaccurate (misleading in up to 40% of cases) due to non-functioning incidentalomas or microscopic bilateral nodular hyperplasia. In surgical candidates > 35 years of age, bilateral adrenal venous sampling (AVS) with cortisol-corrected aldosterone measurement is the gold standard required to prove lateralization of autonomous aldosterone hypersecretion before performing unilateral adrenalectomy.