This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Disorders of Adrenal Gland, Subject name: Medicine
Which is the most common variant of congenital adrenal hyperplasia? 
Select and explain. 
A. 11b hydroxylase deficiency, B. 17a hydroxylase deficiency, C. 3b hydroxyl-steroid dehydrogenase, D. 21 hydroxylase deficiency
D: 21 hydroxylase deficiency
Answer: D: 21 hydroxylase deficiency 
Explanation: Mutations in CYP21A2 which leaads to 21 hydroxylase deficiency is the most prevalent cause of CAH responsible for 90-95% of cases.