Which vitamin is produced when the skin is exposed to sunlight?

When the skin is exposed to sunlight, specifically ultraviolet B (UVB) rays, it engages in a biochemical process that results in the production of vitamin D. This vitamin is crucial for maintaining healthy bones and supporting the immune system. The skin synthesizes previtamin D3 from 7-dehydrocholesterol, which is then converted to vitamin D3 (cholecalciferol) through thermal isomerization. This vitamin D3 can then be further processed by the liver and kidneys into its active form, calcitriol.

Other vitamins mentioned, such as vitamin C, A, and K, do not follow this mechanism of synthesis through sun exposure. Vitamin C is primarily obtained through dietary sources and is not synthesized in the body, while vitamin A can be sourced from dietary retinol or provitamin A carotenoids, and vitamin K is also sourced from dietary intake and gut bacteria. Therefore, the correct answer is vitamin D, as it is the only vitamin produced in significant amounts through skin exposure to sunlight.