Which statement best describes the vitamin D metabolic sequence?

• A. Sun exposure directly yields active vitamin D
• B. Vitamin D is produced in the liver before the skin
• C. The liver alone activates vitamin D
• D. The full sequence is UV exposure in skin → liver conversion to 25(OH)D → kidney activation to 1,25(OH)2D

Answer: (D)

Vitamin D activation happens in two steps after skin synthesis. UVB exposure in the skin converts 7-dehydrocholesterol to vitamin D3, which is then carried to the liver and hydroxylated to 25-hydroxyvitamin D. This form travels to the kidney, where 1α-hydroxylase adds another hydroxyl group to produce 1,25-dihydroxyvitamin D, the active hormone. The sequence in which skin produces the precursor, the liver makes 25(OH)D, and the kidney activates it to the 1,25(OH)2D form is essential, because sun exposure alone does not yield the active vitamin D, and the liver alone cannot activate it without the renal step. 25(OH)D is the major circulating reservoir, while 1,25(OH)2D is the hormonally active form that increases intestinal calcium absorption and supports bone health.