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NBME 21 Answers

nbme21/Block 1/Question#47 (56.3 difficulty score)
A 26-year-old woman (III-2) comes to the ...
50% in females but near 0 in males๐Ÿ”

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submitted by drmantistoboggan4(17),
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tI dias ti saw taafl to smael in ,ruteo and teh iustenoq sekda uboat vile rbno piffnosr.g cSeni the slaem treโ€™na ignbe rbno in the sirft eclpa, I dias %05 easlmef dan 0% sm.ale

hungrybox  fuck i got baited +30  
jcrll  "live-born offspring" โ† baited +21  
sympathetikey  Same :/ +  
arkmoses  smh +  
niboonsh  why is it 50% females tho? +2  
imgdoc  felt like an idiot after i figured out why i got this wrong. +1  
temmy  oh shit! +  
suckitnbme  This isn't exactly right as males can still be born as evidenced by individuals III 6,9,11. This basically an x-linked recessive disease. A carrier mother can still pass her normal X chromosome to a son (50% chance). It's just that the other 50% chance of passing an affected X chromosome results in death of the fetus in utero. Thus all males actually born will not be affected. +2  
makinallkindzofgainz  @suckitnbme, Correct, but if you're a live-born male, you 100% for sure do NOT have the disease, so the chance of a live-born male "being affected" is 0. +3  
spow  @suckitnbme it's not X-linked recessive, otherwise every single son would be affected and therefore have died in utero. It's X-linked dominant +2  
qball  Jail-baited +  

submitted by nwinkelmann(284),
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idePeger = XLD ot(n all goeasternin ffatceed = eXd,li-nk etsffac masel nad mflasee asyiillmr =

ecdteAff streafh = %100 siansnosmrti ot ,edsgurhta 0% inaritsomnss ot ssn.o
A eftfcde strmeoh = 50% to hesdutr,ga 0%5 isnroitnsams ot .ohBston s satpnre cefdatef = 10%0 stmssaiinrno to deasghrtu (due ot sf'trhea X emshoco,ro)m %50 niostsmiarns ot sosn due( ot to'hsmre X rocsmm .oeoh)hotB ptsaner ceafedtf each nmtttgsanrii thob to hrguaetd y(somhzguoo hgtd)ruea = %50 and rmoe eevesr.

If dtiooicnn is ymnoluifr atlfa ot asmle ni ruote, etnh het %05 fetdacfe deasb on oanminirsst as( eoabv) illw eid ni uo,rte and het %50 tno ecaetffd liwl aehv liev .tirhbs Tihs ,nmsea irks of laemef inebg tcfadfee = 0%5 nad kirs of o-irvlnbe asmle gbine efeatdfc = .0%

divakhan  I believe its not XLD, had it been there would still be 25% chance of males to be normal according to the Punnett square. (FA 2020 Page 59. +  
divakhan  ^ 50% chance of being normal.. which is NOT in the answer, it said 0% chance in males! +  
divakhan  Plus you'd see disease in EVERY generation if it was Dominant. For it to be XLD, the father I,1 would have transmitted it to his daughter. Instead the parents are carrier in Gen I. so its XLR disease (II, 3 being a homozygous XLR female) +  
plzhelp123  This is X-linked dominant (think Rett syndrome), if it were x-linked recessive, a female would need to have 2 affected x chromosomes to be symptomatic, which would mean her father would HAVE to be symptomatic as he has only one X chromosome, which is impossible as he would have perished in-utero. The reason the answer is 0 in males is because the question asks specifically about live-born males. If it had asked what is the risk of the fetus being affected then it would be 50% in males (but that 50% would not survive to birth) +2  

submitted by ragacha(14),
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submitted by tsp(0),
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sI het wnmao ni teh firts enaegitorn 2)(1 tno efeatcfd escubea fo eopnimclte 1cptnae?e)re2(n has ot have teh tiatr for hte asieeds ot delevpo in the ufeurt genoniaer.t

b1ackcoffee  disease developed due to germline mosaicism (mutation in of the oocyte) +  

submitted by divakhan(12),
This is not XLD, its XLR disease (FA 2020 page 59)

Here, the mother is homozygous for the mutation therefore she displays the disease.

We see that,

  1. It skips generations
  2. Males are more severely affected (die in utero)
  3. Females are affected (only when homozygous)

Here we see, that mother can pass defective X chromosome to 50% females & other 50% will be carriers (healthy X chromosome from father, defective from mother) For males, Mother passes defective X chromosome so they die (only 1 X chromosome)

For the males who are living (III 6,11 shown in pedigree), they have been lucky to survive due to mosaicism! ;)

b1ackcoffee  don't you think you are going through too many hoops (assumptions)? male survive due to mosaicism? Better chance is disease started due to germline mosaicism and it IS XLD. btw the disease is incontinentia pigmenti (not necessary to know). your second last para doesn't make sense, read again with fresh mind and perspective. +1