This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Pediatrics
[{'content': 'In congenital adrenal hyperplasia most common deficiency \nSelect and explain. \nA. 21- alpha hydroxylase deficiency, B. 11- beta hydroxylase deficiency, C. 17 - hydroxylase deficiency, D. 3 beta hydroxylase deficiency', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans) a (21- alpha ) Ref Nelson 18'h ed p 2360More than 90% of CAH cases are caused by 21 -hydroxylase deficiency.", 'role': 'assistant'}]