This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of lipid, Subject name: Biochemistry
Which is the first intermediate in the conversion of cholesterol to bile acids? 
Select and explain. 
A. Ergosterol, B. Lanosterol, C. 7- hydroxycholesterol, D. 7- dehydrocholesterol
C: 7- hydroxycholesterol
Answer: C: 7- hydroxycholesterol 
Explanation: 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 cofactors. It is the rate-limiting step in the synthesis of bile acids.