Active, saturable transcellular calcium absorption that is upregulated by calcitriol occurs mainly in which segment, using which intracellular binding protein?
- A Terminal ileum, using transcobalamin II
- B Duodenum, using calbindin-D9k ✓
- C Colon, using calreticulin
- D Jejunum, using ferritin
Correct answer: B. Duodenum, using calbindin-D9k
Explanation
The duodenum is the principal site of hormonally regulated calcium entry. Calcitriol induces TRPV6 channels on the apical membrane and calbindin-D9k in the cytosol, which buffers calcium and shuttles it to the basolateral Ca-ATPase. Passive paracellular absorption occurs along the rest of the small intestine but is not vitamin D dependent. Transcobalamin relates to A12 transport, ferritin to iron storage, and neither is involved in calcium handling.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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