<prompt> Active form of Vitamin D ?CholecalciferolCalcifediolCalcitriol25-hydroxy cholecalciferol  <response>: Ans. is c i.e., CalcitriolVitamin D Vitamin D is the major vitamin involved in the absorption of calcium.Beside dietary intake, cutaneous synthesis is the other and more impoant source of vitamin D cholecalciferol in the body. The keratinocytes of stratum corneum of the epidermis of skin contain 7-dehydrocholesterol which is conveed to Vitamin D3 cholecalciferol by the action of sunlight UV light.This vitamin D3 Cholecalciferol is conveed to calcidiol calcifediol or 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3 by 25 hydroxylase in the liver.Calcidiol is the major circulating form of vitamin D3.Calcidiol is then conveed to Calcitriol 1-25 dihydroxy-cholecalciferol or 1-25 dihydroxy vitamin D3 by 1-a hydroxylation in kidney.The final hydroxylation in kidney is rate limiting step.Calcitriol is the active form of vitamin D.Dietary vitamin D is absorbed in ileum and requires bile salts.When Ca2 level is high, little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24, 25 dihydroxycholecalciferol instead. 