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We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

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Embryology: 5-alpha reductase… MCQ (B7-ANAT6-42)

Q: What is the answer to this Anatomy question?

The best answer is D, 5-alpha reductase type 2 deficiency, so dihydrotestosterone was not available to virilise the external genitalia in utero. Dihydrotestosterone, formed from testosterone by 5-alpha reductase 2, is needed to virilise the urogenital sinus and genital tubercle.

Q: What concept does this question test?

It tests Embryology: 5-alpha reductase deficiency in Anatomy. Testosterone alone can maintain the mesonephric duct derivatives, so internal male structures form, and the external genitalia look female at birth. At puberty a surge of testosterone produces virilisation.

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Anatomy · Embryology: 5-alpha reductase deficiency

Sample item

A 16-year-old raised as a girl has primary amenorrhoea. She is 46,XY with testes in the inguinal canal, a normal testosterone level with a raised testosterone to dihydrotestosterone ratio, no uterus and a blind vagina. At puberty she developed voice deepening and clitoral enlargement. Which defect is most likely?

A46,XY gonadal dysgenesis, so no testicular tissue developedBAndrogen receptor defect, so tissues cannot respond to testosterone or dihydrotestosteroneCAnti-Müllerian hormone receptor defect, so the paramesonephric ducts persistD5-alpha reductase type 2 deficiency, so dihydrotestosterone was not available to virilise the external genitalia in utero

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. 5-alpha reductase type 2 deficiency, so dihydrotestosterone was not available to virilise the external genitalia in utero

Dihydrotestosterone, formed from testosterone by 5-alpha reductase 2, is needed to virilise the urogenital sinus and genital tubercle. Testosterone alone can maintain the mesonephric duct derivatives, so internal male structures form, and the external genitalia look female at birth. At puberty a surge of testosterone produces virilisation.

Option by option

A. 46,XY gonadal dysgenesis, so no testicular tissue developed
Gonadal dysgenesis gives streak gonads with a uterus and no puberty-related virilisation.
B. Androgen receptor defect, so tissues cannot respond to testosterone or dihydrotestosterone
Complete androgen insensitivity gives no virilisation at puberty and often breast development.
C. Anti-Müllerian hormone receptor defect, so the paramesonephric ducts persist
A defect in anti-Müllerian hormone signalling gives a uterus and tubes in a male phenotype.
D. 5-alpha reductase type 2 deficiency, so dihydrotestosterone was not available to virilise the external genitalia in utero (correct)
Correct. Absent dihydrotestosterone conversion explains female external genitalia with later virilisation at puberty.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Embryology: 5-alpha reductase deficiency: practice question

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