A 10-day-old female infant presents with vomiting, poor feeding, dehydration, and hyperkalemia with hyponatremia. Examination shows ambiguous genitalia with clitoromegaly. Serum 17-hydroxyprogesterone is markedly elevated. Which enzyme deficiency explains these findings?

• A 11-beta hydroxylase
• B 17-alpha hydroxylase
• C 3-beta hydroxysteroid dehydrogenase
• D 21-beta hydroxylase ✓

Explanation

21-hydroxylase deficiency is the commonest form of congenital adrenal hyperplasia. Blocked conversion of 17-hydroxyprogesterone to 11-deoxycortisol shunts precursors into androgen synthesis, causing virilization, while loss of cortisol and aldosterone produces the salt-wasting crisis seen here. 11-beta hydroxylase deficiency also virilizes but causes hypertension from deoxycorticosterone excess, which contradicts the hypotensive salt-wasting picture, killing option A.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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