A neonate is found to have ambiguous genitalia. Karyotype is 46,XX. Serum 17-hydroxyprogesterone (17-OHP) is markedly elevated at 35,000 ng/dL. Serum sodium is 120 mEq/L, potassium 6.8 mEq/L. This pattern is caused by deficiency of which enzyme?

• A 11-beta-hydroxylase
• B 3-beta-hydroxysteroid dehydrogenase
• C 17-alpha-hydroxylase
• D 21-hydroxylase (CYP21A2) ✓

Explanation

The classical salt-wasting form of congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is the most common cause (>90%) of CAH. It blocks cortisol and aldosterone synthesis, causing accumulation of 17-OHP and shunting of precursors into the androgen pathway, leading to virilization in 46,XX females. Hyponatremia and hyperkalemia reflect mineralocorticoid deficiency (aldosterone deficit). 11-beta-hydroxylase deficiency also raises androgens but causes hypertension due to accumulation of 11-deoxycorticosterone (a mineralocorticoid), not salt-wasting.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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