Most common cause of Congenital adrenal hyperplasia is deficiency of:

ANSWER: (B) 21 alpha hydroxylaseREF: Nelson Textbook of Pediatrics 17th edition page 1909"More than 90% of CAH cases are caused by 21-hydroxylase deficiency"See APPENDIX-58 below for "Congenital adrenal hyperplasia" APPENDIX - 58Congenital Adrenal HyperplasiaCAH is very frequently asked in pre PG examinations with different case scenarios. There is a simple way to remember the symptomatology and reach to the conclusion of the deficient enzyme.Enzyme deficientAldosteroneTestosterone11 (if 1 is to increase, 11 is to both testosterone and aldosterone increases)| (salt retention, hypertension)|Precocious puberty in malesAmbiguous genitalia in females17 (1 is to increase so only aldosterone increases)| (salt retention, hypertension)|Ambiguous genitalia in malesPrecocious puberty in females21 & 3 (1 is to increase so only testosterone increases)| (salt wasting, hypotension)|Precocious puberty in malesAmbiguous genitalia in femalesLipoid| (salt wasting, hypotension)|Ambiguous genitalia in malesPrecocious puberty in femalesSymptoms as per the increase or decrease of aldosterone and testosterone will be:f Aldosterone = salt retention & hypertension (irrespective of the gender)I Aldosterone = salt wasting & hypotension (irrespective of gender)t Testosterone = Ambiguous genitalia in females, precocious puberty in malesI Testosterone = Ambiguous genitalia in males, precocious puberty in females