<prompt> Most common type of Congenital adrenal hyperplasia CAH?21-hydroxylase deficiency11-b hydroxylase deficiency3-b hydroxylase deficiency17-a hydroxylase deficiency  <response>: Ans. a 21-hydroxylase deficiency Ref OP Ghai 7th p. 492; H-17thp. 2267.The 21-hydroxylase deficiency is the commonest form of CAH, accounting for 90 of all cases.Type of CAHFeatures1.21-hydroxylase deficiency Most common cause of ambiguous genitalia at birth Salt-losing form of CAH Episodes of acute adrenal insufficiency with hyponatremia, hyperkalemia, dehydration, and vomiting. These infants and children often crave salt. Deficits in both cortisol and aldosterone secretion.2.11-hydroxylase deficiency Ambiguous genitalia Hypertensive form of CAH Diagnosis is confirmed by demonstrating increased levels of 11-deoxycortisol in the blood or increased amounts of tetrahydro-11-deoxycortisol in the urine.3.3b hydroxylase deficiency Very high levels of urine DHEA with low levels of pregnanetriol and of cortisol metabolites in urine are characteristic. Marked salt-wasting may also occur.4.17 a-hydroxylase deficiency Female or ambiguous. CAH Hypertension due to corticosterone 11-deoxycorticosterone, except in isolated 17,20-lyase deficiency.5.Adults with late-onset adrenal hyperplasia partial deficiency of CYP21A2, CYP11B1, or 3-HSD2 Characterized by normal or moderately elevated levels of urinary 17-ketosteroids and plasma DHEA sulfate. A high basal level of a precursor of cortisol biosynthesis such as 17-hydroxy- progesterone, 17-hydroxypregnenolone, or 11-deoxycortisol, or elevation of such a precursor after ACTH stimulation, confirms the diagnosis of a partial deficiency. Measurement of steroid precursors 60 min after bolus administration of ACTH is usually sufficient. Adrenal androgen output is easily suppressed by the standard low-dose 2 mg dexamethasone test. 