A 50-year-old woman with primary hyperparathyroidism has serum calcium of 12.5 mg/dL and is found to have a urine calcium of 150 mg/day (low-normal for her hypercalcemia). PTH directly acts on which nephron segment to increase calcium reabsorption, and through which mechanism?
- A Both B and C are correct ✓
- B Thick ascending limb; via paracellular calcium reabsorption driven by lumen-positive voltage
- C Distal convoluted tubule; via TRPV5 apical calcium channel upregulation
- D Proximal tubule; via NHE3 stimulation
Correct answer: A. Both B and C are correct
Explanation
PTH stimulates calcium reabsorption in both the thick ascending limb (by enhancing NKCC2 activity, increasing the lumen-positive voltage that drives paracellular Ca²⁺ reabsorption) and the distal convoluted tubule (by upregulating TRPV5 apical channels and calbindin-A28k). Distractor D is wrong because proximal tubule calcium reabsorption is largely passive and paracellular, not directly PTH-regulated via NHE3.
Reference: Boron and Boulpaep Medical Physiology, 3rd ed.
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