Approximately what percentage of congenital adrenal hyperplasia cases are due to 21-hydroxylase deficiency?

• A. 50%
• B. 30%
• C. 70%
• D. 95%

Answer: (D)

The main idea: 21-hydroxylase deficiency is by far the most common cause of congenital adrenal hyperplasia. This enzyme defect, usually due to CYP21A2 mutations, accounts for the vast majority of CAH cases—roughly 90–95% in most references, with many sources rounding to about 95%. Because it represents the overwhelming majority, this option best matches what is seen clinically and in exam literature. The other percentages understate how common 21-hydroxylase–deficient CAH is, while 70% is still lower than the typical estimate.