A female neonate presents with virilized external genitalia (Prader stage III) at birth. Serum 17-hydroxyprogesterone is markedly elevated. Cortisol is low. Aldosterone is undetectable and renin is high. The enzyme deficient in this most common form of congenital adrenal hyperplasia is:
- A 21-alpha-hydroxylase — salt-losing form with hyperkalemia, hyponatremia, and virilization ✓
- B 11-beta-hydroxylase — salt-losing with hypertension
- C 3-beta-hydroxysteroid dehydrogenase — presents with male genital ambiguity
- D 17-alpha-hydroxylase — presents with female phenotype in XY individuals
Correct answer: A. 21-alpha-hydroxylase — salt-losing form with hyperkalemia, hyponatremia, and virilization
Explanation
21-hydroxylase deficiency accounts for >95% of all CAH cases. It blocks conversion of 17-OHP to 11-deoxycortisol (cortisol pathway) and progesterone to 11-deoxycorticosterone (aldosterone pathway). The classic salt-losing form (75% of 21-OHD) presents with cortisol and aldosterone deficiency plus androgen excess. Females are virilized at birth (Prader staging). Males appear normal but enter adrenal crisis in the first 2 weeks. Elevated 17-OHP is the diagnostic marker. Treatment is hydrocortisone + fludrocortisone + salt supplementation in infancy.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.