NBME Physiology 3

Question 1

An arterial blood gas shows pH 7.28, PaCO2 28 mmHg, HCO3- 13 mEq/L. What is the primary disorder?

Accepted Answer

Metabolic acidosis with respiratory compensation

Answer

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?

Accepted Answer

Proximal convoluted tubule

Answer

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:

Accepted Answer

Increasing Na+ reabsorption and K+ secretion

Answer

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:

Accepted Answer

Freely filtered, not reabsorbed, and not secreted

Answer

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:

Accepted Answer

Loss of HCl from gastric secretions

Answer

Vomiting removes HCl, depleting H+ and raising serum HCO3-, resulting in metabolic alkalosis with contraction of ECF volume.

NBME Practice Test