<prompt> Most active form of Vitamin D -7-dehydrocholesierol25-hydroxy-cholecalciferol1, 25dihydroxvcholecalciferol24, 25 dihydroxycholecalciferol  <response>: 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 Ca2 level is high, little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24,25 dihydroxycholecalciferoi instead. 