Which of the following is a potassium-sparing diuretic?

• A. Spironolactone
• B. Furosemide
• C. Hydrochlorothiazide
• D. Acetazolamide

Answer: (A)

Potassium-sparing diuretics work by reducing potassium loss in the distal nephron. Spironolactone blocks the mineralocorticoid (aldosterone) receptor in the collecting ducts, preventing aldosterone from increasing sodium reabsorption and potassium secretion. The net result is modest diuresis with potassium retained, which is why this drug is classified as potassium-sparing and useful in conditions with excess aldosterone.

In contrast, the other diuretics promote potassium loss. A loop diuretic inhibits the Na-K-2Cl transporter in the thick ascending limb, causing substantial sodium, water, and potassium excretion. A thiazide diuretic inhibits the Na-Cl cotransporter in the distal tubule, also enhancing potassium loss. A carbonic anhydrase inhibitor in the proximal tubule leads to bicarbonate and fluid loss and tends to cause potassium wasting.