A 45-year-old man with recurrent calcium oxalate kidney stones is found to have hypercalciuria on 24-hour urine collection. Thiazide diuretics are prescribed for stone prevention. The mechanism by which thiazides reduce urinary calcium excretion is:
- A Inhibition of calcium reabsorption in the thick ascending limb
- B Enhanced calcium reabsorption in the distal convoluted tubule due to volume contraction ✓
- C Increased intestinal calcium absorption via vitamin D activation
- D Direct inhibition of bone resorption
Correct answer: B. Enhanced calcium reabsorption in the distal convoluted tubule due to volume contraction
Explanation
Thiazides inhibit the Na-Cl cotransporter in the distal convoluted tubule, causing natriuresis and volume contraction. This activates proximal tubular sodium and calcium reabsorption, reducing urinary calcium excretion. The enhanced basolateral Na/Ca exchange in the distal tubule also contributes. Loop diuretics increase calcium excretion by inhibiting reabsorption in the thick ascending limb, which is why they are not used for stone prevention.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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