NBME 17 25% (NBME Answers)

Retired NBME 17 Answers

nbme17/Block 0/Question#0 (reveal difficulty score)
A 45-year-old woman comes to the physician ...
25% 🔍 / 📺 / 🌳 / 📖
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submitted by tinyhorse(12)

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Frankly pretty floored that anybody thought that this question contained enough information for someone to confidently answer it.

The question has you assume that both parents are heterozygotes at the locus. Why? I assume I'm missing some esoteric fact about P450 allele frequencies.

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flapjacks I got lucky guessing the same % chance that siblings share HLA markers +5

baja_blast I agree with OP seriously no idea how anyone could have gotten this right without totally guessing it. Am I missing something here?? +2

snow_6 Personally I got this question wrong, partially due to me panicking in this question. Looked through it later and understood the following: - the pt. is homozygous i.e. AA; meaning each parent is heterozygous for the allele - if her sister is to have the same alleles, she is also homozygous i.e. AA - do the punnet square for Aa (mom) and Aa (dad) to get a homozygous allele of AA for only 25% of the time - based on the assumption that parents are heterozygous because question doesn't state any other extra information Only if i could've thought like this during the actual exam +

sschulz2013 However, if one parent is homozygous and the other is heterozygous, then it works out that the sister will have the same allele 50% of the time, not 25%. So still not sure about the above answer. Don't think assuming the parents are both heterozygotes is something we should have to do. There is probably more to it than just that. +1

j44n the probability for being the same gene as a sibling thats homozygous is always 25% this has been on just about every practice NBME +1

chaosawaits Because neither of the parents have history breast cancer, it is assumed that neither parent is homozygous. +5

kavarthapuanusha I assumed neither of the parents have it ,so if one child has it has to be like autosomal recessive pattern , even though breast cancer isnt AR, the patient is homozygous , so the chance of sister also being homozygous is 1/4 = 25% +

+4 upvote downvote
submitted by ∗lpp06(41)

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CYP's are highly polymorphic genes, so it's likely that there are 4 different alleles in the 2 parents, making for 4 possible different combinations in the offspring and a 25% chance of each one occurring.

Agreed that the question is a bit nit-picky because it requires you to know that CYP is a polymorphic gene. However you can back into the reasoning by combining 2 ideas: CYP's function is drug metabolism + there is a wide spectrum of how quickly patients metabolize drugs (ie fast, medium, slow acetylators). From there you can make the wide arrange of phenotypes must mean that there is a wide number of alleles.

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kevinsinghkang That's true but the sister had confirmed breast cancer too so wouldn't it be more than 25% in this case? Or should we just ignore the fact that she had breast cancer too +

thatmd the sister has breast cancer but was treated with tamoxifen, which means she doesn't have this cyt p450 that causes the reduced active form of the drug. So the question is not about breast Ca +1

+2 upvote downvote
submitted by ∗joshua(3)

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UWorld explains it well. Uworld QID: 14781

HLA genes are clustered within a short region of a single chromosome. This results in a low rate of crossover, allowing the HLA gene cluster to be treated as a HLA calotype. Each child inherits 2 halotypes, one from the mother and one from the father. Therefore, the probabilities that a given sibling will share some or all of the same HLA genes are as follows:

1/4 chance of inheriting all the same HLA genes (identical match)
1/2 chance of inheriting half of the same genes (haploidentical HLA match)
1/4 chance of inheriting none of the same HLA genes (HLA mismatch)

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neoamin BUt this question is not about HLA mate ^^ +2

melanoma I think is the same concept +2

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