Which vitamin is synthesized by the skin upon exposure to sunlight?

The vitamin synthesized by the skin upon exposure to sunlight is Vitamin D. When the skin is exposed to ultraviolet (UV) rays from the sun, it converts a cholesterol derivative known as 7-dehydrocholesterol into previtamin D3. This, in turn, is transformed into active vitamin D (calcitriol) through further processes in the liver and kidneys.

Vitamin D plays an essential role in calcium metabolism and bone health, helping to maintain adequate serum calcium and phosphate levels. It also has implications for immune system function and may assist in reducing inflammation.

The other options represent different vitamins that do not rely on sun exposure for their synthesis. While Vitamin A can be obtained from dietary sources as well as converted from carotenoids in the body, it is not synthesized in the skin through sunlight. Vitamin K is essential for blood clotting and bone metabolism, but it is obtained primarily from dietary sources and not produced by the skin. Similarly, Vitamin E acts as an antioxidant and is also sourced mainly through food intake rather than through sun exposure. Therefore, the correct answer, which highlights the specific relationship between sunlight and Vitamin D synthesis in the skin, is indeed Vitamin D.