NBME 21 46,XY (NBME Answers)

Retired NBME 21 Answers

nbme21/Block 4/Question#20 (reveal difficulty score)
A 7-month-old boy is brought to the physician ...
46,XY 🔍 / 📺 / 🌳 / 📖
tags:

+19 upvote downvote
submitted by ∗hungrybox(1277)

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This video explains genital embryology extremely well.

If you felt totally lost like me, watch the video first at 2x, then check out the bottom figure on pg. 608 in FA 2019.

Relevant to this question:

SRY proteins stimulate development of testes
This pt has testes => he must have the SRY gene on the Y chromosome
MIF degrades the Mullerian duct, which would otherwise become the internal female genitalia
This pt has internal female genitalia => didn't make enough MIF

Quiz yourself (answers in a separate post):

This patient's disorder can be traced back to which cells?
This patient had normal genitalia. If this patient had smaller genitalia than normal, that would be a defect in what enzyme?

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ergogenic22 I like to work backwards. 1) patient has normal testicles on histology, normal appearing penis this must mean a Y chromosome is present, as testis determining factor is on the y chromosome (see above post point #2). I.e. you can eliminate choice A and B. Theoretically, 47XY and 47XYY could also present with female genitalia due to lack of MIF, but normal 46XY is more common +13

+7 upvote downvote
submitted by ∗hungrybox(1277)

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quiz yourself answers:

Sertoli (Sertoli Shuts down, MIF is secreted by Sertoli cells)
5-alpha reductase

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lovebug 5-alpha reductase is due te that DHT is important for male external genitalia? +

+5 upvote downvote
submitted by ∗d_holles(218)

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this is a classic description of persistent Mullerian duct syndrome

wikipedia.org/Mullerian_duct_syndrome

due to lack of MIF (mutation in the gene that makes MIF) → persistence of the Mullerian duct → uterus and fallopian tube present in a XY male w/ testes (typically cryptorchid).

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fatboyslim I don't think the problem is due to undescended testes since sexual differentiation occurs earlier than testicular descent (7 weeks gestation vs. 33 weeks gestation). +

+4 upvote downvote
submitted by ∗drachenx(72)

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Because his testicles are undescended, his sertoli cells are probably not working properly, and therefore not synthesizing MIF correctly. Which is why his internal female genitalia is allowed to develope in a default setting.

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+3 upvote downvote
submitted by gabstep(15)

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I'm thinking this is decreased levels of anti-Mullerian hormone? This is what usually inhibits the paramesonephric duct (ultimately develops into internal female genitalia) in males. A patient can be XY and still have decreased levels.

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gabstep FA 2019 pg 608 +

+2 upvote downvote
submitted by ∗readit(18)

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Addition to Hungrybox's explanation:

Why it's not 47,XXY:

This would be referring to Klinefelter's, which is characterized by small, poorly functioning testicles.

However, question states "morphologic studies of a biopsy specimen of the testes show no abnormalities"

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readit More specifically, in Klinefelter's, you would see "hyaline seminiferous tubules/sertoli cells" on histology +3

pg32 Thanks! Where did you get the histologic description from? +1

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