Which statement best describes physiological calcium absorption in the small intestine?
- A It occurs exclusively in the terminal ileum by a receptor-mediated pathway
- B Active transcellular transport in the duodenum uses TRPV6 uptake and cytosolic calbindin, and is upregulated by 1,25-dihydroxycholecalciferol ✓
- C It requires simultaneous absorption of intrinsic factor-bound cobalamin in the jejunum
- D It proceeds entirely by passive paracellular diffusion independent of vitamin D status
Correct answer: B. Active transcellular transport in the duodenum uses TRPV6 uptake and cytosolic calbindin, and is upregulated by 1,25-dihydroxycholecalciferol
Explanation
The duodenum is the main site of regulated calcium uptake: luminal Ca2+ enters through apical TRPV6 channels, binds calbindin-D9k to buffer free intracellular Ca2+, and exits via the basolateral Ca2+-ATPase. 1,25(OH)2D induces all three components. Paracellular calcium absorption also exists in the jejunum but is not the regulated route.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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