Name the vitamin that is mainly synthesized in the skin upon exposure to sunlight.

Vitamin D is synthesized in the skin when it is exposed to ultraviolet (UV) radiation from sunlight. This process involves the conversion of 7-dehydrocholesterol, a compound present in the skin, into previtamin D3, which is then transformed into the active form of vitamin D through further heat and chemical processes within the body.

Vitamin D plays a crucial role in calcium absorption, bone health, and immune function. Its synthesis through sunlight exposure is particularly significant, as dietary sources alone often do not provide enough of this vitamin for optimal health. Unlike some vitamins that are primarily obtained from food sources, vitamin D is unique in its natural synthesis triggered by sunlight, making this a key aspect of its importance in nutrition and health.