USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Which enzyme deficiency leads to the masculinization of female infants and maternal virilization during pregnancy?

1. Aromatase deficiency

2. 17-alpha hydroxylase deficiency

3. 5-alpha reductase deficiency

4. 17-beta hydroxysteroid dehydrogenase deficiency

The correct answer is: Aromatase deficiency

Aromatase deficiency would cause an increase in testosterone levels, which can lead to the masculinization of female infants and maternal virilization during pregnancy. It is a plausible response as it is an enzyme that plays a key role in the biosynthesis of estrogen. Option B is incorrect as 17-alpha hydroxylase deficiency would lead to decreased levels of testosterone, resulting in female-typical genitalia. Option C, 5-alpha reductase deficiency, may also seem like a potential answer since it is involved in the conversion of testosterone to its more potent form dihydrotestosterone (DHT), but it would not result in an excess of testosterone and therefore would not cause masculinization in female infants. Option D, 17-beta hydroxysteroid dehydrogenase deficiency, can result in ambiguous genitalia, but it would not necessarily lead to the specific manifestations described in the question. Therefore, the correct answer is A.