This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cholesterol synthesis and bile acids, Subject name: Biochemistry
[{'content': 'Which of the following is the rate-limiting enzyme of bile acid synthesis? \nSelect and explain. \nA. microsomal 7-a hydroxylase, B. mitochondrial 7-a hydroxylase, C. microsomal 17-a hydroxylase, D. mitochondrial 17-a hydroxylase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: microsomal 7-a hydroxylaseis rate-limiting enzyme of bile acid synthesis The first intermediate in the synthesis of bile acids is 7a-hydroxycholesterol It is catalyzed by the enzyme 7a-hydroxylase, a microsomal CYP450 enzyme, which requires NADPH, oxygen, and vitamin C as co-factors.', 'role': 'assistant'}]