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NBME 24 Answers

nbme24/Block 2/Question#42

A 3-week-old female newborn is brought to the ...

The patient has another mutation that was not included in the previous analysis

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 +5  upvote downvote
submitted by colonelred_(46),

The analysis only showed a mutation in one allele. CF is an autosomal recessive disease: the disease only manifests if there are mutations in both alleles of the CFTR gene.

If you still have 1 functional copy of the CFTR gene, you can still make the CFTR protein (the chloride channel/transporter), hence your body won’t have any issues.

This is analogous to tumor suppressor genes like Rb: so long as one of the alleles you have is functional, you can make enough of the protein to “make up” for the defective allele. If both get knocked out (Rb-/-), you lose the protection provided by the gene because now you make no protein at all.

The only thing that made sense for this question was the fact that the other allele was not included in the analysis.

charcot_bouchard  OR another allele has a diff type of mutation because CF is done by like hundreds of diff type of mutation. SO the 70 types that we screened covered one type from one parent but not another that was inherited from other parent. +4  
soph  I put D thinking there was a mutation in another protein that interacts with CFTR....thus u dont have CF but some disease with similar phenotype. Is this wrong bc its simply not the case ?? +  
nbmehelp  @charcot_bouchard I think that makes more sense if I understand what you're saying- Probably had a mutation only in 1 of 2 of the same alleles in the analysis but had another mutation in 2 of 2 alleles at a different location not included in the analysis, right? +  
fallot4logy  CF is a rare disease , and the possibility to have a mutated gene plus a gene that its not belong to 70 most common cf mutations is extremely rare +  




 +1  upvote downvote
submitted by eli_medina9(1),

ALLELIC HETEROGENEITY (FA page 57 2019)

Theres multiple allele variants for the CFTR gene in a single locus, so you could get cystic fibrosis from a mutation in any one of those allele variants(theres over 1500 different mutations described) the question stem mentioned they tested for the most common types, so we can assume they probably just missed testing for mutations in other alleles.





 -1  upvote downvote
submitted by sweetmed(37),

you need a mutation in 2 alleles to get CF [since it is autosomal recessive]/





one does not simply “walk into morder”