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NBME 21 Answers

nbme21/Block 1/Question#30

A newborn has female external genitalia and a 46,XY ...


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Male internal genitalia -> Intact SRY , testes, and testosterone.

No female internal genitalia -> Presence of MIF (antimullerian hormone) and intact Sertoli cell function.

Female external genitalia -> No androgen present, which is required for male external genitalia formation.

d_holles  Not sure I understand why T is wrong, but DHT is correct. +  
d_holles  I thought about this some more -- DHT forms external genitalia while T forms 'male genital ducts'. That's why the correct answer is DHT, not T, since the PT had +ext genitalia, but -internal genitalia. I was thinking that the PT had CAIS, but that would lead to testes only w/o male genital ducts. See FA2019 p608. +  
d_holles  *I meant -ext genitalia, +int genitalia +  
adong  T is wrong because you still need T to make the internal male organs which he has based off the MRI +  

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submitted by mcl(277),

Per p608 in FA 2019, SRY on Y chromosome results in development of testes. DHT results in development of male external genitalia (and the prostate).

mrsmac  No sertoli cells or lack of mullerian inhibitory factor makes more sense. bc there is both male and female internal genitalia but only male external genatalia. and karyotype would show 46XY. First Aid 2018 pg. 604 - the "Sexual Differentiation" charge delineates exactly this. If it were 5areductase deficiency the child would have testicles and scrotum, which in this case is absence. Hope this makes sense. Please let me know if you disagree and why. Thanks. +  
mixmasta  I believe the tricky part is that they don't mention the status of the Male external genitalia. Pg. 605 from FA ( bottom portion) shows the external development of the Male/Female genitalia; you see DHT is need for male. Furthermore, pg. 604 (SEXUAL DIFFERENTIATION) DHT is also needed for Male external development. +  
niboonsh  My understanding of this is that the diagnosis is 5alpha reductase deficiency because the newborn has female external (aka ambiguous) with male internal (aka "male genital ducts"). According to FA, leydig cells produce testosterone, which can either stimulate the mesonephric duct to form the INTERNAL male genitals (as see in the pt). Testosterone can also be acted on by 5alpha reductase to become Dihydrotestosterone, which forms the male EXTERNAL genitalia. Since this kid has "female" genitals, but has male insides and is 46XY, id say this is a simple case of 5alpha reductase deficiency. No sertoli cells or no MIF would present as both female and male internal (because MIF typically inhibits differentiation of female internal) and male external genitalia (bcuz leydig cells are unaffected) +8  

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submitted by yo(28),

Dang this question threw me off because it said female genitalia when I thought it would be ambiguous at birth. DHT still makes the most sense. Thanks.

Everything has been covered already, but here's to the visual learners - it's easy to see the relationship between DHT and testosterone. https://imgur.com/a/M8Y3Fdm

(ignore the whole black versus blue colors...it complicates things and I'm sure I miscolored somewhere).