Where does 25-hydroxyvitamin D convert to 1,25-dihydroxyvitamin D under normal renal function?

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Multiple Choice

Where does 25-hydroxyvitamin D convert to 1,25-dihydroxyvitamin D under normal renal function?

Explanation:
The activation of vitamin D occurs in the proximal tubule, where the enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol). This process is stimulated by parathyroid hormone, especially when calcium is low, and serves to increase intestinal absorption of calcium and phosphate. The distal tubule, glomerulus, and collecting duct do not perform this conversion, so under normal renal function the proximal tubule is the site of formation for the active vitamin D.

The activation of vitamin D occurs in the proximal tubule, where the enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol). This process is stimulated by parathyroid hormone, especially when calcium is low, and serves to increase intestinal absorption of calcium and phosphate. The distal tubule, glomerulus, and collecting duct do not perform this conversion, so under normal renal function the proximal tubule is the site of formation for the active vitamin D.

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