Ans. is 'd' i.e.. Lungso This question is a repeat from AI -06.o Following organs are involved in calcium metabolism through vitamin D:-i) Skin- Synthesis of 7-dehydrocholesterol which is converted to cholecalceferol (vitamin D,) by sunlight.ii) Liver- Converts cholecalceferol to calcifediol by enzyme 25-hydroxylase.iii) Kidney- Converts calcifediol to calcitriol, active form of vitamin D.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 D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skinr 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 liverr.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 kidneyro The final hydroxylation in kidney is rate limiting stepr.o Calcitriol is the active form of vitamin Dr.o Dietary- vitamin D is absorbed in ileum r 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 insteadr.