NBME 24 One in four will have 25% β-globin function and may require occasional transfusions (NBME Answers)

Retired NBME 24 Answers

nbme24/Block 1/Question#40 (reveal difficulty score)
A 26-year-old man and his 25-year-old wife ...
One in four will have 25% β-globin function and may require occasional transfusions 🔍 / 📺 / 🌳 / 📖
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+31 upvote downvote
submitted by ∗brethren_md(105)

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Make a punnett square with a cross of B B+ and B B0; B+ represents 50% function while B0 represents 0% (null) function. So in this case, the husband would have a B B0 genotype while the wife has a B B+ genotype.

Cross of these two will result in the following genotypes; BB, BB0, BB+, B+B0 BB = 100% function, BB+ = 75% function, BB0 = 50% function, B+B0 = 25% function

So the answer will be 1 in 4 have a 25% function given the genotypes.

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tyrionwill how about the choice of D: 1 in 4 have 50% function, which is true. shall 50% function needs transfusion? +

tyrionwill In FA, it defines beta thalassemia into minor (HbA2 >3.5%) and major (both HbF and HbA2 go further up), and the major needs transfusion frequently. How can we take this classification based upon quantitive way like in this question? how much percentage of function left does not need a frequent transfusion? +

azibird D says one in TWO, not one in FOUR. +6

hemehero Is there a way to know that B+B0 will = 25% function. I was stuck between 25% function and 10% function, but couldn't figure out how to reason between the two of them. +3

neil_simmons The question says the mother has a mutation known to cause 50% decrease in beta-globin gene function of one allele. So if one allele is working at 50% (B+) and the other allele is working at 0% (B0), then that would mean that particular set of alleles would function at 25%. +2

twich22 The Key here is the woman has 50% decrease in function "Of one allele". Its a stupid way to word it, But it means that one allele works at 50%, which would be B+. So if you have a B0/B+ offspring, the B0 gives 0% and the B+ works at 50% capacity, so you only have 25% of normal. +

murad some one explain this to me please, how come if the mother with his genotype BB+ has 50% function of one allele. while if we get her offspring punnet square we consider the one that has the same genotype as her BB+ of having 75% function?? +

+3 upvote downvote
submitted by ∗an_improved_me(91)

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I feel like its these kinds of questions that really make a test-taker succeed. You could have studied your whole life for Step 1, and then...come up against a question like this. Its testing concepts that you're familiar with (e.g. Beta-thalassemia, genetics), but in a way that isn't something you'd ever directly learn. You have to stay calm, analyze what you know, what the question is asking, create a strategy to answer, and look at which answer is best.

I envy some of you!!

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+1 upvote downvote
submitted by ∗medstruggle(21)

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Whats the difference between “heterozygous null mutation in B globin gene” and “heterozygous mutation known to cause 50% decrease in B globin gene function of one allele”?

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welpdedelp I interpreted "null" as meaning full deletion while the other heterozygous mutations was only a 50% decrease in the function. One child would inherit 1 null mutation and 1 50% mutation, which would leave them with a 25% functional gene. +18

j44n A thal is the deletion, B thal is mutations on promoters or splice sites. +

-1 upvote downvote
submitted by mynamejeff(0)

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I think E is also viable... 1/4 is B/B and another 1/4 is B/B+ . The B/B+ is B-thalassemia minor where B chain is underproduced (functions normally) and is usually asymptomatic, per FA (2020 pg. 418).

Thoughts?

Also, my understanding on B-thalassemia from Dr. Sattar and first aid is that it's not set in stone that B+ is 50% production. It seems to vary.

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syringomyelia1 E says 1/2 not 1/4 +1

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