This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetic and genetic disorders, Subject name: Pediatrics
Most common enzyme deficiency in congenital adrenal hyperplasia is 
Select and explain. 
A. 3 beta hydroxylase, B. 11 beta hydroxylase, C. 17 hydroxylase, D. 21 alfa hydroxylase
D: 21 alfa hydroxylase
Answer: D: 21 alfa hydroxylase 
Explanation: congenital adrenal hyperplasia is a group of autosomal recessive defects in steroid synthesis characterised by deficiency of adrenocoical hormones The commonest form of congenital adrenal hyperplasia accounting for about 90% of cases is due to deficiency of 21-alpha hydroxylase It is associated with diminished synthesis of coisol and aldosterone Reference: Ghai TB of pediatrics 8th edition pg 526