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Retired NBME 15 Answers

nbme15/Block 4/Question#29 (reveal difficulty score)
A 45-year-old woman (III,7) comes to the ...
Incomplete penetrance ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—hungrybox(1277)
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A: Gonadal mosaicism | Present in child, not parent โ†’ would not have family history of disease

B: Incomplete penetrance | Correct! Half of children affectd, skips a generation โ†’ AD inheritance likely.

C: Nonpaternity โ†’ Prader-Willi

D: Somatic mosaicism | Present in parent, not child โ†’ would not have family history of disease

E: Variable expressivity | Affected patients have varying disease severity โ†’ Rule out b/c mother is unaffected

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cassdawg  Also, nonpaternity can be a way of saying that the assumed biological father is not actually the father (can be a case of artificial insemination or cheating, etc.). +4
beto  In genetics, a non-paternity event is when someone who is presumed to be an individual's father is not in fact the biological father. +1



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submitted by โˆ—mittelschmerz(49)
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How do you know for sure that this is incomplete penetrance and not gonadal mosaicism? Dont both allow an AD disease to be transmitted by a phenotypically non-expressing carrier?

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nissimhazkour1  my line of thinking is that gonadal mosaicism is much less likely considering there is a family history of the disease. If there was no family history then a gonadal mutation causing mosaicism is possible, but taking into consideration how there is a clear AD inheritance, it must be that the person inherited the disorder but is not expressing the phenotype. hope this helps! +3
mittelschmerz  Yes thanks! That feels like it should have been so obvious in retrospect, ugh. +1
i_hate_it_here  Don't sweat it fam, I hate it here too +5



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