This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins, Subject name: Biochemistry
[{'content': 'Rate limiting step in vitamin D synthesis is? \nSelect and explain. \nA. 25 cholecalciferol, B. 1, 25-dihydroxycholecalciferol, C. 24, 25- dihydroxycholecalciferol, D. 7-dehydrocholesterol', 'role': 'user'}, {'content': 'Answer: B \nExplanation: ANSWER: (B) 1, 25-dihydroxycholecalciferolREF: Ganong 22ed chapter 21, Harrison 17 " edition chapter 21 Vitamin-D: Synthesis and Metabolism1, 25-dihydroxyvitamin D is the major steroid hormone involved in mineral ion homeostasis regulationVitamin D is subsequently 25-hydroxylated in the liver by cytochrome P450-like enzymes in the mitochondria and microsomes. The activity of this hydroxylase is not tightly regulated, and the resultant metabolite, 25-hydroxyvitamin D , is the major circulating and storage form of vitamin DThe second hydroxylation, required for the formation of the mature hormone, occurs in the kidney. The 25-hydroxyvitamin D-l-hydroxylase is a tightly regulated cytochrome P450-like mixed function oxidase expressed in the proximal convoluted tubule cells of the kidney', 'role': 'assistant'}]