Which vitamin is produced in the skin via sunlight exposure and is essential for calcium absorption?

• A. Vitamin C
• B. Vitamin A
• C. Vitamin K
• D. Vitamin D

Answer: (D)

Vitamin D is produced in the skin when exposed to sunlight and is essential for calcium absorption. UVB light converts a compound in the skin (7-dehydrocholesterol) into vitamin D3, which is then processed in the liver and kidneys to its active form, calcitriol. Calcitriol increases the intestines' ability to absorb calcium (and phosphate), supporting bone mineralization and overall calcium balance. Other vitamins have different roles and are not produced by sunlight in a way that drives calcium uptake—Vitamin C supports collagen and antioxidant defense, Vitamin A affects vision and cell function, and Vitamin K is important for blood clotting and bone remodeling but does not mediate calcium absorption via sunlight-derived vitamin D.