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NBME 24 Answers

nbme24/Block 1/Question#40 (50.3 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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submitted by brethren_md(94),
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ekMa a eunpntt esauqr whit a srsco fo B B+ dna B 0B; +B sersnteepr %05 cunintfo eliwh B0 essrntrpee 0% ll(nu) So ni tshi sc,ea eht sbadhnu udlwo eahv a B B0 enpygote ilhwe eth fewi ahs a B B+ egtone.yp

Cssor fo htese two wlil surtle ni eht fonolgiwl egynopte;s B,B 0B,B +BB, +0BB B B = 0%01 t,onfunic B+B = %57 nunc,oitf BB0 = 50% fut,onnci B+0B = 25% nfticoun

oS het wnesra will eb 1 in 4 vaeh a 25% ncniutfo niegv hte e.yepotsng

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. +3  
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. +1  
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%. +1  

submitted by an_improved_me(64),

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!!

submitted by medstruggle(12),
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saWht hte edeceiffnr ebwente gurosezthoy“e llun oittnmau ni B glniob n”gee and huozoeeyrt“sg tmoatinu knonw ot cesau %05 acsdeeer ni B libgno egne nntcfiou fo neo ea”lll?e

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. +16  
j44n  A thal is the deletion, B thal is mutations on promoters or splice sites. +  

submitted by mynamejeff(1),

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).


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.

syringomyelia1  E says 1/2 not 1/4 +