This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Active form of Vitamin D ? \nSelect and explain. \nA. Cholecalciferol, B. Calcifediol, C. Calcitriol, D. 25-hydroxy cholecalciferol', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is 'c' i.e., CalcitriolVitamin D Vitamin D is the major vitamin involved in the absorption of calcium.Beside dietary intake, cutaneous synthesis is the other and more impoant source of vitamin D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skin contain 7-dehydrocholesterol which is conveed to Vitamin D3 (cholecalciferol) by the action of sunlight (UV light).This vitamin D3 (Cholecalciferol) is conveed to calcidiol (calcifediol or 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3) by 25 hydroxylase in the liver.Calcidiol is the major circulating form of vitamin D3.Calcidiol is then conveed to Calcitriol (1-25 dihydroxy-cholecalciferol or 1-25 dihydroxy vitamin D3) by 1-a hydroxylation in kidney.The final hydroxylation in kidney is rate limiting step.Calcitriol is the active form of vitamin D.Dietary vitamin D is absorbed in ileum and requires bile salts.When Ca2 level is high, little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24, 25 dihydroxycholecalciferol instead.", 'role': 'assistant'}]