A newborn girl develops vomiting, dehydration, and hyponatraemic hyperkalaemic dehydration in the second week of life. Labs show 17-hydroxyprogesterone 12000 ng/dL (markedly elevated), low cortisol, low aldosterone, and high renin. Karyotype is 46,XX. What is the enzyme defect?
- A 21-hydroxylase deficiency ✓
- B 17-alpha hydroxylase deficiency
- C 11-beta hydroxylase deficiency
- D 3-beta hydroxysteroid dehydrogenase deficiency
Correct answer: A. 21-hydroxylase deficiency
Explanation
Classic salt-wasting congenital adrenal hyperplasia from 21-hydroxylase deficiency presents in the neonatal period with salt wasting (hyponatraemia, hyperkalaemia) and virilisation in a 46,XX infant. Massively elevated 17-hydroxyprogesterone is the diagnostic hallmark because the blocked pathway shunts substrate proximal to 21-hydroxylation. 11-beta hydroxylase deficiency accumulates 11-deoxycortisol and causes hypertension, not salt wasting. 17-alpha hydroxylase deficiency presents with hypertension and sexual infantilism, not neonatal crisis.
Reference: Nelson Textbook of Pediatrics, 22nd ed.
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