<prompt> Which of the following is rate limiting step -25 hydroxylation of vitamin D24 hydroxylation of vitamin D1 hydroxylation of vitamin DNone  <response>: Ans. is c i.e., 1 hydroxylation of vitamin D 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.o 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 calcidiol 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3 by 25 hydroxylases in the liver.o Calcidiol is the major circulating form of vitamin D3.o Calcidiol 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 stepQ.o Calcitriol is the active form of vitamin DQ.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.o 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 D, Cholecalciferol is converted to calcidiol 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3 by 25 hydroxylase in the liver.o Calcidiol is the major circulating form of vitamin D3o Calcidiol is then converted to calcitriol 1-25 dihydroxy-cholecalcifero! or 1-25 dihydroxy vitamin D3 by 1-a hydroxyiation in kidneyQ.o The final hydroxy iation in kidney is rate limiting stepQ,o Calcitriol is the active form of vitamin DQ. 