This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
[{'content': 'The most common cause of ambiguous genitalia in a newborn is: \nSelect and explain. \nA. 21 hydroxylase deficiency, B. 11b -hydroxylase deficiency, C. 17a -hydroxy alse deficiency, D. 3 b-hydroxysteroid deficiency', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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, "Nelson\'s Textbook of Pediatrics", Volume 2, 18th Edition, New Delhi, Pages 2360-2364', 'role': 'assistant'}]