This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Pediatrics
In congenital adrenal hyperplasia most common deficiency 
Select and explain. 
A. 21- alpha hydroxylase deficiency, B. 11- beta hydroxylase deficiency, C. 17 - hydroxylase deficiency, D. 3 beta hydroxylase deficiency
A: 21- alpha hydroxylase deficiency
Answer: A: 21- alpha hydroxylase deficiency 
Explanation: Ans) a (21- alpha ) Ref Nelson 18'h ed p 2360More than 90% of CAH cases are caused by 21 -hydroxylase deficiency.