Correct Answer: C. 21 hydroxylase deficiency
21-hydroxylase deficiency accounts for ~90% of congenital adrenal hyperplasia (CAH) cases in India and worldwide. The enzyme defect blocks cortisol synthesis at the 17-hydroxylase step, causing shunting of precursors toward androgen production. In a 46,XX female fetus, excess fetal androgens (androstenedione → testosterone) during weeks 8–12 of gestation cause virilization of external genitalia, presenting as clitoromegaly, labial fusion, or ambiguous genitalia at birth. The karyotype 46,XX confirms genetic female with normal chromosomal sex but phenotypic virilization due to androgen excess. This is the classic presentation of salt-wasting or simple virilizing CAH. The diagnosis is confirmed by elevated 17-hydroxyprogesterone (17-OHP) on newborn screening or postnatal testing. Management includes glucocorticoid (hydrocortisone) and mineralocorticoid (fludrocortisone in salt-wasting form) replacement, and prenatal dexamethasone in future pregnancies to suppress fetal ACTH and prevent virilization.
Why the other options are wrong
A. 2,3 Iyase deficiency — This is not a recognized enzyme in steroid synthesis pathways. This is a distractor with a non-existent or misnamed enzyme. No CAH form presents with clitoromegaly due to a '2,3 iyase' deficiency. This option tests whether students confuse real enzyme names in the adrenal pathway. B. 11 hydroxylase deficiency — 11β-hydroxylase deficiency is the second most common CAH form (5–8% of cases) and does cause virilization in 46,XX females. However, it characteristically presents with hypertension and hypokalemia due to accumulation of 11-deoxycortisol and deoxycorticosterone (mineralocorticoid effect), whereas 21-hydroxylase deficiency presents with salt-wasting or simple virilization without hypertension. The question lacks mention of hypertension, making 21-hydroxylase more likely. **D. 3 beta dehydrogenase deficiency** — 3β-HSD deficiency causes CAH but is rare (1% of cases) and presents with mild virilization in 46,XX females and salt-wasting. More distinctively, it causes undervirilization in 46,XY males (due to impaired testosterone synthesis), which is the opposite of the typical male presentation. The classic presentation of clitoromegaly in a 46,XX child without mention of male undervirilization points away from this enzyme defect.
High-Yield Facts
- 21-hydroxylase deficiency accounts for ~90% of CAH cases; presents with virilization in 46,XX females (clitoromegaly, labial fusion) and salt-wasting or simple virilizing forms.
- Elevated 17-hydroxyprogesterone (17-OHP) is the diagnostic marker for 21-hydroxylase deficiency; measured on newborn screening or postnatal testing.
- Virilization in 46,XX CAH occurs during weeks 8–12 of gestation due to excess fetal androgens (androstenedione → testosterone) from shunted precursors.
- 11β-hydroxylase deficiency (second most common CAH) causes virilization + hypertension/hypokalemia; 21-hydroxylase causes virilization ± salt-wasting without hypertension.
- Prenatal dexamethasone (0.5 mg TDS) suppresses fetal ACTH and prevents virilization in future pregnancies of affected mothers; requires early initiation (before week 6).
- Postnatal management of CAH: hydrocortisone (glucocorticoid) + fludrocortisone (mineralocorticoid in salt-wasting form); dose adjusted by 17-OHP and clinical response.
Mnemonics
CAH Enzyme Defects & Presentation (VIRIL-CAH) Virilization (46,XX) → 21-hydroxylase (90%) or 11β-hydroxylase (8%); Infertility/undervirilization (46,XY) → 3β-HSD; Rare forms → lipoid CAH (P450scc); Increased 17-OHP → 21-hydroxylase; Low cortisol + high ACTH → all forms of CAH. 21-Hydroxylase vs 11β-Hydroxylase (SALT rule) 21-OH deficiency: Salt-wasting, Androgen excess, Low BP (or normal), Typical (90% of CAH). 11β-OH deficiency: Hypertension, Increased DOC, Good virilization, Hypokalemia.
NBE Trap
NBE pairs virilization in a 46,XX child with multiple enzyme deficiencies to test whether students reflexively choose the rarest form (3β-HSD) or the second most common (11β-hydroxylase) instead of recognizing that 21-hydroxylase deficiency accounts for 90% of CAH and is the most likely diagnosis in a straightforward virilization case without hypertension.
Clinical Pearl
In Indian pediatric practice, 21-hydroxylase deficiency is the leading cause of ambiguous genitalia in newborn girls and is screened on national newborn screening programs. Early recognition and prenatal counseling for future pregnancies (with dexamethasone) can prevent virilization and improve long-term outcomes including fertility and psychosocial well-being.
_Reference: OP Ghai (Pediatrics) Ch. Endocrine Disorders; Harrison Ch. 405 (Congenital Adrenal Hyperplasia); Robbins Ch. 24 (Endocrine System)_