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
Which of the following is the rate limiting enzyme of bile acid synthesis 
Select and explain. 
A. Mitochondrial 17 α hydroxylase, B. Cytoplasmic 7 α hydroxylase, C. Microsomal 7 α hydroxylase, D. Mitochondrial 7 α hydroxylase.
C: Microsomal 7 α hydroxylase
Answer: C: Microsomal 7 α hydroxylase 
Explanation: Microsomal 7 α hydroxylase (cholesterol 7 α hydroxylase) is the rate limiting enzyme of bile acid synthesis. This enzyme is expressed only in liver.