Describe the vitamin D metabolic pathway from sun exposure to the active hormone calcitriol.

• A. Skin 7-dehydrocholesterol to vitamin D3 via sunlight; liver 25-hydroxylase forms 25(OH)D; kidney 1α-hydroxylase forms 1,25(OH)2D (calcitriol) under PTH regulation.
• B. Skin produces vitamin D2; liver forms 25(OH)D; kidney forms calcitriol directly.
• C. Skin to cholesterol to vitamin D via sunlight; liver and kidney are not involved.
• D. Skin 7-dehydrocholesterol to vitamin D3; kidney 25-hydroxylase forms 25(OH)D; liver 1α-hydroxylase forms calcitriol.

Answer: (A)

Sunlight converts the skin’s 7-dehydrocholesterol into vitamin D3 (cholecalciferol). This vitamin D3 then travels to the liver, where a 25-hydroxylase adds a hydroxyl group to form 25-hydroxyvitamin D [25(OH)D], the main circulating form. From there, the kidney uses 1α-hydroxylase to convert 25(OH)D into 1,25-dihydroxyvitamin D [calcitriol], the active hormone. The production of calcitriol in the kidney is stimulated by parathyroid hormone, especially when calcium is low. Calcitriol increases intestinal absorption of calcium and phosphate, supporting bone mineralization and maintaining calcium balance. The liver and kidney steps are essential; without them, calcitriol cannot be produced. Vitamin D2 from plants can contribute, but the classic pathway from sun exposure relies on vitamin D3 produced in the skin, followed by liver 25-hydroxylation and kidney 1α-hydroxylation to form the active hormone.