Most active form of Vitamin D -

Ans. is 'c' i.e., 1, 25 dihydroxycholecalciferoi o Vitamin D is the major vitamin involved in the absorption of calcium.o Beside dietary intake, cutaneous synthesis is the other and more important source of vitamin D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skin contain 7-dehydrocholesterol which is converted to Vitamin D3 (cholecalciferol) by the action of sunlight (UV light),o This vitamin D3 (Cholecalciferol) is converted to calcidioi (calcifcdiol or 25 hydroxy cholecalciferol or 25- hydroxy vitamin D3) by 25 hydroxylase in the liver,o Calcidioi is the major circulating form of vitamin D.o Calcidioi is then converted to Calcitriol (1-25 dihydroxy-cholecalciferol or 1-25 dihydroxy vitamin D3) by 1-a hydroxylation in kidney.o The final hydroxylation in kidney is rate limiting step,o Calcitriol is the active form of vitamin D.o Dietary vitamin D is absorbed in ileum and requires bile salts.o When Ca+2 level is high, little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24,25 dihydroxycholecalciferoi instead.