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
Commonest cause of congenital adrenal hyperplasia is 
Select and explain. 
A. 21 hydroxylase deficiency, B. 11 B-hydroxylase deficiency, C. 17 alpha-hydroxylase deficiency, D. 18 hydroxylase deficiency
A: 21 hydroxylase deficiency
Answer: A: 21 hydroxylase deficiency 
Explanation: (21 hydroxylase) (489-90-G) (492-Ghai 7th)* 21 hydroxylase & 11-b hydroxylase deficiency are common varieties21 hydroxylase11-b hydroxylase deficiency* May present as salt-wasting form of CAH, due to the associated aldosterone deficiency resulting in life threatening emergencies in the early postnatal life* These may present with severe vomiting and vascular collapse* Boys with CAH do not have anomaly of the external genitals at birth* Results in accumulation of deoxycortisol (DOC) because enzymatic block is beyond this stage* These children develop hypertension of varying severity due to excessive DOC