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
The rate-limiting step in the synthesis of bile acids is catalyzed by: 
Select and explain. 
A. 12a-Hydroxylase, B. 7a-Hydroxylase, C. HMG coA reductase inhibitors and bile acid sequestrant, D. 25-hydroxycholecalciferol 1-hydroxylase
B: 7a-Hydroxylase
Answer: B: 7a-Hydroxylase 
Explanation: The rate-limiting step in the formation of bile acids is the 7a-hydroxylation of cholesterol, catalyzed by a microsomal NADPH-dependent enzyme 7a-hydroxylase. 12a-Hydroxylase is also involved in the bile acid synthesis, but is not regulated. HMG CoA reductase and pyro diphosphomevalonate decarboxylase are both cholesterol-synthesizing enzymes; 25-hydroxycholecalciferol 1-hydroxylase is a kidney enzyme, which catalyzes the formation of an active vitamin D3 metabolite, 1,25-di-OH D3. Ref: Lippincott's Biochemistry, 5th Ed, page 241