Which diuretic should be avoided under CrCl < 30 mL/min due to hyperkalemia risk?

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

Answer: (B)

In very low kidney function (CrCl under 30 mL/min), potassium-sparing diuretics pose a high risk of hyperkalemia. Spironolactone is an aldosterone antagonist that reduces potassium excretion in the distal nephron. When the kidneys are already weak, this can lead to dangerous potassium buildup. That’s why it should be avoided in CrCl < 30 mL/min. Other diuretics listed do not carry the same hyperkalemia risk: loop diuretics like furosemide promote potassium loss; thiazides like hydrochlorothiazide also tend to cause potassium loss and are less effective with very low GFR; mannitol is not used chronically and isn’t a potassium-sparing agent.