Exposure to UV light in the skin leads to the production of which vitamin D form?

• A. Vitamin D2
• B. Calcitriol
• C. Vitamin D3
• D. Vitamin D1

Answer: (C)

UVB exposure in the skin triggers a chemical change in 7-dehydrocholesterol, converting it into pre‑vitamin D3, which then becomes vitamin D3 (cholecalciferol). This is the form that the skin actually synthesizes in response to sunlight. From there, vitamin D3 is transported to the liver for hydroxylation to calcidiol and then to the kidney to form the active calcitriol, but the initial product created by UV light is vitamin D3 itself. The other forms aren’t produced directly by UV exposure in the skin: vitamin D2 comes from plant sources, calcitriol is the active hormone produced after metabolic processing, and vitamin D1 isn’t a standalone vitamin D form produced by the skin.