For nephrolithiasis due to idiopathic hypercalciuria, which diuretic reduces urinary calcium excretion?

• A. Ethacrynic acid
• B. Dorzolamide
• C. Hydrochlorothiazide
• D. Triamterene

Answer: (C)

The key idea is to lower calcium in the urine to prevent stone formation in idiopathic hypercalciuria. Thiazide diuretics do exactly that: they act on the distal tubule to promote calcium reabsorption back into the bloodstream. This happens because inhibiting the Na-Cl transporter there reduces sodium reabsorption, and the resulting physiological changes favor calcium uptake into the body rather than loss in urine. The mild volume depletion they cause also helps the kidney reclaim more calcium. The net effect is a decrease in urinary calcium.

Other diuretics don’t have this benefit. Loop diuretics increase calcium excretion, which would worsen hypercalciuria. Carbonic anhydrase inhibitors mainly affect bicarbonate handling and overall urine flow, not calcium reabsorption. Potassium-sparing diuretics don’t reliably reduce urinary calcium either. So the diuretic that best reduces urinary calcium—and helps prevent stones in this scenario—is a thiazide like hydrochlorothiazide.