What is the COMMONEST cause of primary amenorrhea with ambiguous genitalia in a female with 46XX chromosome?

21 hydroxylase deficiency is the most common cause of androgenization in chromosomal 46 XX females. In this condition there is a block in the synthesis of glucocoicoid and mineralocoicoid and increase in the level of 17 hydroxyprogesterone. Increased androgen synthesis in utero causes androgenization of the female fetus in the first trimester. At bih, there is ambiguous genitalia with varying degrees of clitoral enlargement and labial fusion. Enzyme deficiency Glucocoicoids Mineralocoicoids Androgens Other effects 21 b OH lase low low increased Masculinization of females in utero Incomplete precocious pubey Adrenal hyperplasia Hyponatremia and hypovolemia 11 b OH lase low low aldosterone high DOC increased Mineralocoicoid excess (DOC) Hypervolemia and hypernatremia 17 a OH lase low low aldosterone high DOC very low DOC excess Absent secondary sexual characters Amenorrhea Ref: Achermann J.C., Jameson J.L. (2012). Chapter 349. Disorders of Sex Development. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.