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nbme24/Block 2/Question#42 (51.9 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_(107),
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eTh ysaslian ynol whoesd a aiunotmt in eon el.eall FC si an ualasotom reveiscse :siedeas eth dassiee lyno enamftiss if eerth ear imuaonstt ni both llaslee of teh TCRF .egne

If yuo sllti heva 1 caloifnnut yocp of teh RFTC eeg,n ouy nca tlsil akme eht TFCR eotrinp the( hcdeoilr r/nsaaetrctnhl)eornp, neceh ouyr yobd otw’n vhae yna iessu.s

hTsi is ngsouloaa to ormtu rsuspopers gnese elki bR: so onlg as one fo eth allslee ouy ehav is t,lcaoiunnf uoy cna ekma eoghun fo the opnetir ot am“ek ”pu ofr the itceeefdv .aellle fI hbto gte ekcknod out --,R)(b/ ouy elso eht iprcntooet vodirdpe by hte neeg sueecba wno oyu meka no opnetir at la.l

heT only htnig ttha mdae sesen ofr htis touesiqn saw eth fatc htta eht ertho lealel saw nto ndecdliu in the lya.siasn

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. +40  
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 ?? +10  
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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reeTsh pilumtle lelael srianatv rof hte TRFC gene ni a lsgein uclso, so yuo colud tge cycsti sfboisri from a aiottumn ni yna noe of oshte leella svrit(aensretha over 1500 feetdnifr osittamnu dbi)rceeds het qsiutone smte doemntnei hety tesedt ofr eht tsom mmoocn y,tspe os ew anc sesuma etyh rbpylabo sujt idssem gtetsin rfo tnotimusa ni oreht .elellas

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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oyu dene a monuaitt ni 2 elaesll to gte FC snie[c it is toasmoaul ceie]/srsve

submitted by an_improved_me(18),

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.