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: Biochemistry
[{'content': 'Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamine D3. What is the rate limiting enzyme in Vitamin D synthesis? \nSelect and explain. \nA. 25 cholecalciferol, B. 1, 25-dihydroxycholecalciferol, C. 24, 25- dihydroxycholecalciferol, D. 7-dehydrocholesterol', 'role': 'user'}, {'content': "Answer: B \nExplanation: Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamine D3. 25D is the 'storage' form of vitamin D and is used to determine the vitamin D status of individuals. The last and rate limiting step in the synthesis of 'active' 1,25D is catalyzed by the mitochondrial enzyme 1 hydroxylase and is conventionally known to take place in kidneys.", 'role': 'assistant'}]