NBME Physiology 3 — Questions and Answers
Question 1: An arterial blood gas shows pH 7.28, PaCO2 28 mmHg, HCO3- 13 mEq/L. What is the primary disorder?
- Metabolic acidosis with respiratory compensation (Correct answer)
- Respiratory acidosis with metabolic compensation
- Metabolic alkalosis
- Mixed respiratory and metabolic alkalosis
Correct answer: Metabolic acidosis with respiratory compensation
Low pH with low HCO3- indicates metabolic acidosis; the low PaCO2 reflects appropriate respiratory (Kussmaul) compensation.
Question 2: Where is the majority (~80–85%) of filtered bicarbonate reabsorbed?
- Thick ascending limb
- Proximal convoluted tubule (Correct answer)
- Distal convoluted tubule
- Collecting duct
Correct answer: Proximal convoluted tubule
The proximal tubule reabsorbs ~80–85% of filtered HCO3- via carbonic anhydrase-dependent H+ secretion and HCO3-/Na+ cotransport.
Question 3: Aldosterone's primary effect on the collecting duct principal cells is:
- Increasing aquaporin-2 insertion
- Increasing Na+ reabsorption and K+ secretion (Correct answer)
- Decreasing Na+/K+-ATPase activity
- Inhibiting ENaC channels
Correct answer: Increasing Na+ reabsorption and K+ secretion
Aldosterone upregulates apical ENaC and basolateral Na+/K+-ATPase, promoting sodium retention and potassium excretion.
Question 4: Inulin is the gold standard for measuring GFR because it is:
- Freely filtered, partially reabsorbed, and not secreted
- Freely filtered, not reabsorbed, and not secreted (Correct answer)
- Actively secreted and then fully reabsorbed
- Bound to plasma proteins and not filtered
Correct answer: Freely filtered, not reabsorbed, and not secreted
Inulin is freely filtered at the glomerulus and undergoes no tubular reabsorption or secretion, so its clearance equals GFR exactly.
Question 5: A patient with chronic vomiting develops hypokalemia and metabolic alkalosis. The alkalosis is primarily caused by:
- Loss of HCl from gastric secretions (Correct answer)
- Increased renal HCO3- excretion
- Decreased aldosterone activity
- Respiratory compensation for acidosis
Correct answer: Loss of HCl from gastric secretions
Vomiting removes HCl, depleting H+ and raising serum HCO3-, resulting in metabolic alkalosis with contraction of ECF volume.
Question 6: Furosemide produces diuresis primarily by inhibiting which transporter?
- Na+/H+ exchanger in the proximal tubule
- Na+/K+-ATPase in collecting duct
- Na-K-2Cl cotransporter in the thick ascending limb (Correct answer)
- Thiazide-sensitive NaCl cotransporter in the distal tubule
Correct answer: Na-K-2Cl cotransporter in the thick ascending limb
Furosemide blocks the NKCC2 cotransporter in the thick ascending limb, disrupting the countercurrent multiplier and producing copious dilute urine.
Question 7: Antidiuretic hormone (ADH/vasopressin) increases water reabsorption by:
- Inserting aquaporin-1 channels in the proximal tubule
- Inserting aquaporin-2 channels in the collecting duct apical membrane (Correct answer)
- Inhibiting Na+ reabsorption in the distal tubule
- Increasing GFR
Correct answer: Inserting aquaporin-2 channels in the collecting duct apical membrane
ADH stimulates V2 receptors on collecting duct principal cells, triggering cAMP-mediated insertion of aquaporin-2 channels into the apical membrane.
NBME Physiology 3
An arterial blood gas shows pH 7.28, PaCO2 28 mmHg, HCO3- 13 mEq/L.
What is the primary disorder?