A 40-year-old man with recurrent calcium oxalate kidney stones and normal urine volume is advised a thiazide diuretic. The anti-stone benefit of thiazides results from:

• A Inhibition of carbonic anhydrase, raising urine pH
• B Enhanced distal calcium reabsorption leading to hypocalciuria ✓
• C Increased proximal tubular sodium reabsorption, lowering oxalate excretion
• D Direct complexation of calcium in the tubular lumen

Explanation

Thiazides enhance distal convoluted tubular calcium reabsorption, likely by increasing the basolateral Na/Ca exchanger activity secondary to mild volume contraction, producing sustained hypocalciuria and fewer calcium stones. Carbonic anhydrase inhibition would raise urine citrate losses and is not the thiazide action. Thiazides actually increase urinary oxalate concentration slightly, yet the net reduction in calcium supersaturation prevents stone formation.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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