<prompt> Most common cause of Congenital adrenal hyperplasia is deficiency of:17 alpha hydroxylase21 alpha hydroxylase11 beta Hydroxylase deficiency3 beta-HSD deficiency  <response>: ANSWER: B 21 alpha hydroxylaseREF: Nelson Textbook of Pediatrics 17th edition page 1909More than 90 of CAH cases are caused by 21-hydroxylase deficiencySee 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, hypertensionPrecocious puberty in malesAmbiguous genitalia in females17 1 is to increase so only aldosterone increases salt retention, hypertensionAmbiguous genitalia in malesPrecocious puberty in females21 3 1 is to increase so only testosterone increases salt wasting, hypotensionPrecocious puberty in malesAmbiguous genitalia in femalesLipoid salt wasting, hypotensionAmbiguous 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 genderI Aldosterone salt wasting hypotension irrespective of gendert Testosterone Ambiguous genitalia in females, precocious puberty in malesI Testosterone Ambiguous genitalia in males, precocious puberty in females 