A newborn male has ambiguous genitalia. Serum 17-hydroxyprogesterone is markedly elevated. Karyotype is 46,XX. Electrolytes reveal hyponatremia and hyperkalemia on day 5. Which enzyme deficiency is responsible?

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

Explanation

This is congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, the most common form (>90% of cases). The block in cortisol synthesis leads to ACTH-driven accumulation of 17-hydroxyprogesterone (the diagnostic marker) and overproduction of androgens, causing virilization of a 46,XX female (ambiguous genitalia). The salt-wasting form (aldosterone also deficient) presents with hyponatremia, hyperkalemia, and potentially life-threatening adrenal crisis in the first weeks of life. Treatment is hydrocortisone and fludrocortisone.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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