Ans. is "c' i.e., 1,25-dihydroxycholecalciferolVitamin 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 D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skinQ 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 (calcifediol or 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3) by 25 hydroxylase in the liverQ,o Calcidiol is the major circulating form of vitamin D3o Calcidiol is then converted to Calcitriol (1-25 dihydroxy-cholecalciferol or 1-25 dihvdroxy vitamin D3) by 1-a hvdroxylation in kidneyQ,o The final hydroxvlation in kidney is rate limiting stepQ,o Calcitriol is the active form of vitamin DQ.o Dietary vitamin D is absorbed in ileumQ 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 dihydroxycholecalciferol insteadQ.o Calcitriol has following effects : -It enhances absorption of calcium and phosphate from the intestine. It increases the synthesis of calcium binding protein calbindin.Calcitriol has opposite effects on bone, increasing both bone deposition and bone resorption. However, its net effect is increased mineralization of bone. Calcitriol stimulates osteoblasts which cause mineralization of bone. Activated osteoblasts secrete osteoclast stimulating factor which induce development of osteoclasts from their precursors (Osteoclasts themsleves do not have any calcitriol receptorQ). Osteoclastic activation promotes bone resorption.iii) In kidney calcitriol stimulates reabsorption of phosphate in proximal tubule and reabsorption of Ca+2 in the distal tubule.