Question

A 13-year-old girl visits the gynaecology OPD with a complaint of not attaining menarche. Her karyotype is 46,XX. On examination, clitoromegaly is seen. Which enzyme is most likely deficient in this condition?

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

Hint:

Think of the commonest cause of CAH, the one that virilises and salt-wastes.

Answer 1

The Correct Option is A

Step 1: A 46,XX girl with virilisation (clitoromegaly) and primary amenorrhea points to congenital adrenal hyperplasia (CAH).
Step 2: The most common cause of CAH, accounting for more than 90 to 95 percent of cases, is 21-hydroxylase deficiency. Blocked cortisol synthesis drives ACTH up and shunts precursors into androgens, producing virilisation.
Step 3: 11 beta-hydroxylase deficiency also causes virilisation but is far rarer and is accompanied by hypertension. 3 beta-hydroxysteroid dehydrogenase deficiency is rare and causes only mild virilisation.
Step 4: 17 alpha-hydroxylase deficiency does the opposite: it causes lack of androgens, sexual infantilism and hypertension, not clitoromegaly. So the single most likely enzyme is 21 alpha-hydroxylase.
Ref: Dutta Gynaecology, 6th ed, p. 440.