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nbme24/Block 2/Question#42 (51.5 difficulty score)
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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submitted by colonelred_(106),
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eTh iyasanls yonl hwdoes a tinmutao ni eon el.alel FC si na otmsaauol scvreseei ise:saed eht sdeeias only nitassefm if erhte era titoausnm in thob seelall of hte CFRT .egne

If you lstli ahve 1 ntunfoacli oycp fo eth FRCT gnee, oyu cna iltsl mkae eth CRFT nptoire he(t oerilcdh lnrea/nhp)note,rasctr nehce your ydob ’twno heav any seusis.

This is nogalaous to uomrt uoesprprss gsnee klei Rb: os nolg as neo of hte allesle ouy aveh is nulcfto,ani you acn kmae ugonhe fo het tnrieop to mka“e ”pu orf hte fdvieecte leella. fI both get edkknoc uot R-)(/b,- ouy seol eht necoirtotp oriveddp by het eneg eubcesa nwo oyu keam on petnroi ta al.l

heT ynol hnigt htat mdae sesen orf tish eoustniq swa het tcfa thta eth ehrto elaell asw not ucinddle in eth ilsynasa.

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. +39  
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 ?? +9  
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 +4  
gubernaculum  @soph i picked D too but now looking back, the panel had 70 of the most common CFTR gene mutations so it is unlikely that they didn't already check a gene that codes for a protein that interacts with CFTR? that's the only way i can rationalize it. its bad writing ultimately +  
peridot  I also picked D, but there are over 1700 different mutations for CF and it's too hard to test for them all - the panel in the question tested for the 70 most common. As others mentioned, CF is an autosomal recessive disease, so there must be another mutated allele here for the child to present with the disease. It's more likely, and I imagine not uncommon, that the mutation is not in the panel. As for D, I suppose the best reasoning I can come up for it is that nothing like that exists - what protein interacts with ONE mutated CFTR allele in that it results in the same phenotype as CF, a disorder that requires TWO mutated alleles? I have never heard of such a thing, whereas I have definitely heard of A being the case. +2  

submitted by eli_medina9(19),
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Tesher mlptleiu llelae tsraavin ofr het TFRC gnee ni a lsiegn sol,uc so uyo loudc tge ctsicy iiorssbf fmor a oitnutma ni nya eon fo hseot alllee aise(ehtanrrtvs reov 0501 retdnfief itmosnatu )cedibrsed eht etqsinuo emts tdenoemni yteh eetdst ofr teh omts nomomc espty, os ew acn assmeu hety ylorpbba stuj mdiess igsetnt rof ttsminoau in eorht

melanoma  in other alleles?, we only have two alleles per gene +  
qiss  @melanoma "other alleles" as in the same allele with a different type of mutation. Like eli_medina9 mentioned, CF simply can be caused by thousands of different mutations of the CFTR gene. This baby girl has two alleles, one of them with a mutation found in the analysis and the other allele with a mutation that wasn't detected amongst the most common mutations. +  

submitted by bharatpillai(23),

IRT is measured in routine heel-prick blood taken for biochemical screening of all newborn infants born in the UK. This test is one of a number of completed in newborn screening (the "Guthrie Test"). In Australia it is known 94% of those with eventual diagnosis of CF have a positive IRT on newborn screen. Samples with a raised IRT (defined as highest 1% of values) are then screened for common CF gene mutations. Each centre has a slightly different gene panel; currently 40-50 of the most common genes are sequenced. However, there are more than 2000 known mutations, so gene panel testing does miss occasional CF patients

If gene testing finds one mutation they will then have a sweat test to help confirm the diagnosis. Sweat testing is more likely to be equivocal in infants and typically not attempted in those under 5kg. If sweat test is positive more expansive gene testing is considered. If two mutations are found they are diagnosed with CF.

bharatpillai  i swear to god some asshat who wrote this question immediately followed it up by making a wikipedia post about it to pretend like this is some common knowledge medical students were supposed to have. +  

submitted by sweetmed(144),
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you ndee a iattumon in 2 slleael ot get CF ic[sen it is oaosualtm srevc/ei]es

submitted by an_improved_me(16),

Another thing... Isn't it possible that both the sweat chloride test and the immunoreactive trypsin are just false +? Both of these tests have pretty low threshold for returning as a + response. Why can't it be possible that one allelic mutation is enough to cause a positive result in these tests, without actually signifying the pt has a disease? My only guess is that since two answers indicate that there could be a false positive, neither can be the right answer.