<prompt> The most common cause of ambiguous genitalia in a newborn is:21 hydroxylase deficiency11b -hydroxylase deficiency17a -hydroxy alse deficiency3 b-hydroxysteroid deficiency  <response>: More than 90 of congenital adrenal hyperplasia is caused by 21-hydroxylase deficiency. Incidence is 1 in 15000-20000 bihs in most populations. Presents with female pseudohermaphroditism or post natal virilization. Ref: Kliegman, Behrman, Jenson, Stanton, 2007, Chapter 577, Congenital Adrenal Hyperplasia and Related Disorders, In the book, Nelsons Textbook of Pediatrics, Volume 2, 18th Edition, New Delhi, Pages 2360-2364 