Which diuretic is potassium-sparing?

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

Answer: (D)

Potassium-sparing diuretics work by blocking aldosterone’s action in the distal nephron, specifically in the collecting ducts. Aldosterone promotes sodium reabsorption and potassium excretion; by antagonizing its receptor, spironolactone reduces sodium reabsorption and prevents potassium loss, so potassium is retained. That’s why spironolactone is the potassium-sparing option. The other diuretics in the list act differently: furosemide is a loop diuretic that increases excretion of sodium, chloride, and water and tends to cause potassium loss; hydrochlorothiazide is a thiazide that also promotes potassium loss; mannitol is an osmotic diuretic that increases urine flow without a potassium-sparing mechanism.