Which vitamin is primarily synthesized in the skin upon exposure to sunlight?

Vitamin D is primarily synthesized in the skin upon exposure to sunlight because of a biochemical process triggered by ultraviolet B (UVB) rays. When the skin is exposed to sunlight, specifically UVB radiation, it converts 7-dehydrocholesterol, a compound naturally found in the skin, into previtamin D3. This then undergoes thermal isomerization to become vitamin D3 (cholecalciferol).

Subsequently, vitamin D3 is converted to its active form, calcitriol, in the liver and kidneys, playing a crucial role in calcium homeostasis and bone health. This biological synthesis is a unique feature among vitamins and underscores the importance of sunlight for maintaining adequate levels of vitamin D in the body.

Other vitamins listed, such as vitamin C, vitamin A, and vitamin E, cannot be synthesized by the skin through sunlight exposure. Instead, these vitamins are obtained from dietary sources, highlighting the specific and vital role of sunlight in the synthesis of vitamin D.