Calcium absorption in the duodenum occurs predominantly by a saturable transcellular pathway. The correct order of steps in this pathway is:

• A Apical entry through TRPV6 channels, cytosolic diffusion bound to calbindin, basolateral extrusion by Ca2+-ATPase ✓
• B Basolateral Ca2+-ATPase extrusion, cytosolic diffusion bound to calbindin, entry through TRPV6 channels
• C Paracellular flux through tight junctions driven by the lumen-negative potential, independent of vitamin D
• D Apical entry via TRPV6, direct extrusion through the same channel down its concentration gradient

Explanation

In the duodenum, calcium enters enterocytes through apical TRPV6 channels, buffers in the cytosol bound to calbindin-D9k to keep free calcium low, and exits across the basolateral membrane via plasma membrane Ca2+-ATPase. All three proteins are upregulated by calcitriol acting through the vitamin D receptor. Paracellular calcium absorption does occur but predominates in the jejunum and ileum and is not saturable, making option C incorrect for the duodenal pathway.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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