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NBME 21 Answers

nbme21/Block 1/Question#30 (44.2 difficulty score)
A newborn has female external genitalia and a ...
DihydrotestosteroneπŸ”
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 +7 
submitted by sinforslide(49),
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Mela nnaerlit aalenigti &g-;t ncIatt RSY , ,tstees dna est.nrsoeeott

oN fmaeel tlieanrn gietanlia -t&g; ecrePesn of IMF arii(tlleunman omo)hnre and ctnait elitorS lelc tonufc.ni

lFeame rlexanet agianielt ;-> No agdonnre n,etrsep ichwh si drquerie rof alme leaxtner niialaget fani.tromo

d_holles  Not sure I understand why T is wrong, but DHT is correct. +1  
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. +18  
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 +2  



 +1 
submitted by mcl(586),
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ePr 086p ni FA 9021, RYS on Y ceommoohsr sseurtl ni toevlmpndee of stt.ese TDH setsulr ni devlmnteope of meal earxetnl iaagtenil d(an hte )rt.oeaspt

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) +15  



 +1 
submitted by basic_pathology(8),

Y chromosome --> SRY gene --> Testis --> Sertoli and Leydig cells.

Sertoli cells: AMH --> involution of mullerian ducts --> no female internal organs

Leydig cells:

  • Testosterone: development of Wolffian duct (SEED) (seminal vesicles, Epididymis, Ejaculatory Duct, Ductus Deferens).
  • 5a-reductase --> DHT: male external genitalia (penis, prostate)

Everything is normal, except for external genitalia. DHT signalling is missing.




 +0 
submitted by yo(79),
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Dnga iths tnsiqeou ehrtw em ffo causbee it dsai eaeflm aainegtli when I httohug it wludo eb mogasubui ta hbir.t TD H sllit meska het mtso sesn.e Tahn.ks




 +0 
submitted by et-tu-bromocriptine(113),
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yEnghrteiv ahs ebne cdeoerv lya,eadr tub seehr' ot het isvlau leernars - 'its ayse to ees the alnpithoirse ntbewee DTH adn t roseetteso.n/hmrimt3:m8uM./ocapdY/tgs/F

e(gnroi eth hloew acklb sseurv lebu .rcoi.ts.ol cimcetapols ghsnti nda m'I sreu I escmodlior )wesrm.heeo




 -1 
submitted by frijoles(13),

I chose E) because I was thinking androgen insensitivity syndrome. However, I realize the wording of the question is which HORMONE would be reduced and in AIS, testosterone levels would not actually be reduce.

To be clear though, AIS could have the EXACT same described presentation, yes? (46.XY with male genital ducts & female external genitalia) Just want to make sure there is nothing else that could have tipped me off. Thanks.

daddyusmle  Yup you got it. AIS is a defect with the androgen receptor in target tissues, not with the hormones. AIS has the same presentation: Genotypically XY but has female external genitalia and male internal genitalia. This is most likely 5-alpha reductase deficiency. +