Which laboratory test is the accepted standard for assessing total body vitamin D status?

• A Serum 25-hydroxyvitamin D level ✓
• B Serum calcium concentration
• C Serum 1,25-dihydroxyvitamin D level
• D Serum parathyroid hormone level

Explanation

25-hydroxyvitamin D is produced hepatically, circulates bound to vitamin D binding protein with a long half-life of 2 to 3 weeks, and reflects cumulative cutaneous synthesis plus dietary intake, so it is the standard index of vitamin D status. The active hormone 1,25-dihydroxyvitamin D is tightly regulated by PTH and does not fall until deficiency is severe, which kills option C. Serum calcium and PTH change only after secondary hyperparathyroidism is established.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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