This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology metabolism, Subject name: Physiology
The rate-limiting step in the synthesis of cortisol is catalyzed by: 
Select and explain. 
A. 21-Hydroxylase, B. 3b-Hydroxysteroid dehydrogenase, C. Cholesterol side-chain cleavage enzyme, D. 11b-Hydroxylase
C: Cholesterol side-chain cleavage enzyme
Answer: C: Cholesterol side-chain cleavage enzyme 
Explanation: Ans. C. Cholesterol side-chain cleavage enzymeThe first and rate-limiting step in all steroid biosynthesis is catalyzed by cholesterol sidechain cleavage enzyme, resulting in pregnenolone and iso caproic acid. 17a-hydroxylase, 3b-hydroxysteroid dehydrogenase, 21-hydroxylase, and 11b- hydroxylase are all involved in the synthesis of cortisol, but are not rate-limiting. 3-Hydroxy-3-methylglutaryl CoA reductase catalyzes the rate-limiting step in de novo cholesterol synthesis.