Solution:

Vitamin D is a hormone, not a vitamin. The skin is responsible for producing vitamin D. During exposure to sunlight, ultraviolet radiation penetrates into the epidermis and photolyzes provitamin D3 to previtamin D3. Previtamin D3 can either isomerize to vitamin D3 or be photolyzed to lymisterol and tachysterol. Vitamin D is also sensitive to sunlight and is photolyzed to 5,6-transvitamin D3, suprasterol I, and suprasterol II. In Boston, solar irradiation only produces previtamin D3 in the skin