<prompt> Active form of Vitamin D -CholecalciferolCalcifediolCalcitriol25-hydroxy cholecalciferol  <response>: Ans. is c i.e., Calcitriol Vitamin Do 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 D3 cholecalciferol in the body. The keratinocytes of stratum corneum of the epidermis of skin contain 7-dehydrocholesterol which is converted to Vitamin cholecalciferol by the action of sunlight UV light .o This vitamin D3, Cholecalciferol is converted to calcidiol calcifediol or 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3 by 25 hydroxylase in the liver.o Calcidiol is the major circulating form of vitamin D3.o Calcidiol is then converted to Calcitriol 1-25 dihydroxv-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 dihydroxycholecalciferol instead.o This question is direct repeat from NBE based AI 2013-14. 