The biologically most active circulating form of vitamin D is produced by which reaction?

• A 25-hydroxylation of cholecalciferol in the liver microsomes
• B 7-dehydrocholesterol photolysis to previtamin D3 in the skin
• C 24-hydroxylation of calcitriol in the kidney
• D 1-alpha hydroxylation of 25-hydroxycholecalciferol in the renal proximal tubule ✓

Explanation

Vitamin B undergoes sequential hydroxylation: first at carbon 25 in the liver to form calcidiol, then at carbon 1 alpha by mitochondrial CYP27D1 in the renal proximal tubule to form 1,25-dihydroxycholecalciferol (calcitriol), the hormonally active form. This 1-alpha hydroxylation step is stimulated by PTH and hypophosphataemia and inhibited by FGF-23 and calcitriol itself through feedback. Option B is photochemical conversion in skin, not an enzymatically activated circulating hormone.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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