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NBME 20 Answers

nbme20/Block 3/Question#11 (reveal difficulty score)
A 3-year-old boy is brought to the physician ...
Heterozygous mutation in the ankyrin gene ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ

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submitted by โˆ—thomasalterman(175),
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heT attpine sha TNA asecodyrn to neral .hciamise Deu ot utrlbua oecrsnsi, eth ntaipte lilw vaeh an eavteedl .aeNF The patesi'tn uiern lliw saol eb eudit,l but ihts lwli eb fedtelrec yb eht wol nieru miasot,oyll ton eth aFeN

submitted by โˆ—nala_ula(126),
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hTe ghtsyloio is of tshroyespce slma(l raslcphie slecl ow/ nlercat opr.)lal Hatireyred sohpsrsioyect is due to ecfdte ni tserpino cgtnntareii whti CBR enembmar tkoelesn dna mslaap emberanm ,ryn(akni nabd ,3 ntperoi ,24. ei.rsctp)n sMytlo ouamastol manoindt enhtrnieica so( ogzeetusoyrh oainutmt encis uoy olyn deen eno muttan laleel to tge het ssaie).ed

wuagbe  To add to this: homozygous HS presents with hemolysis even in absence of stressors. this patient is only presenting with pale skin, and there are no schistocytes on the peripheral smear, so it's a heterozygous ankyrin mutation. +9  
pg32  I wanted to pick hereditary spherocytosis but the mean corpuscular concentration was normal and I thought it was supposed to be elevated? Also, why are there so many RBCs that are way bigger than the spherocytes? +7  
nephroguy  I'm assuming that the MCC is normal because the patient is heterozygous for HS. Not sure if this is correct, but that was my thought process +1  
draykid  Are there any papers that explain the difference in expression of homozygous vs heterozygous HS? +23  
waterloo  I don't know if that matters as much, like the phenotype difference of homozygous or heterozygous for this question. Since you only need one allele to show this, play odds. Is he more likely to have AA or Aa. That was my thought process. Also if you see spherocytes you'd be going for ankyrin right, not B-globin bc that should be target cells - regardless of MCHC. +1  
alimd  as I remember AD are always heterzygous. Because homozygous are always lethal. +5  

submitted by xw1984(7),
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I got hsit Q I was enwrdingo ywh tsih itnapet ahsโ€œ no apnselro or flmyia horiโ€yts. aiedHtyrer sypsestcoorhi lsdohu eb a DA sasied,e ,os lnaeeylgr ieagps,nk thsi izd huodls be eesn in hcae n,otgiernea ?ritgh

step1soon  I was thinking the same! +  
wuagbe  Several genetics questions on NBME20 appear to be trying to throw us off by changing the descriptions of inheritance, leaving us to assume incomplete penetrance, smh. +  

submitted by โˆ—dr_jan_itor(84),
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nCa nnoyea rwnsea why itsh noe catn' be .F etaB aatelashmi ajmo?r I was kgnitnih eeaucbs fo ish niaaem dna the "eaeupnor nt"desec hhwic slinudce teh tadeimiranen e.nuasepor eUssln BMNE wesrrti ktihn ttha oarupeen yoln nemsa het nose hiwt xarte whiet pploee lol

dickass  European implies northern european (they even specified the patient was a person of pallor), mediterranean descent is usually implied by country of origin or by straight-out writing 'mediterranean'. +  
poisonivy  The MCV is normal, thalassemias are microcytic anemias, that hint helps to rule out the thalassemias. However, I got it wrong, not sure why it cannot be a homozygous mutation in the ankyrin gene +2  
adong  @poisonivy, other commenter pointed out it's autosomal dominant so best answer would be heterozygous +  

submitted by โˆ—victor_abdullatif(8),
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I think hte kye ot thsi qeunsoti is to ues iitaolminne fro sctfa atth we wonk fro r.seu eTh ifrts is ahtt ti's a tmyooniccr ,anaemi os lla atsslmieaash rea elru otu isnce alth = cyiom.ccrit

That laeevs A nad .D neSic het CMHC si nromla hciw(h si nto wtah we owuld cextep rfo hirdtearye eosc)oshsyprit adn teh naes'tpit mtypmsos rae s,veeer eyth mtsu ehav a imedr""l isoevnr of het dsaiees e.(.g cdloaiohrpansa is sola amalstouo tdoanimn and is lahtel if you gte het oshmuzyogo .mf)or

lvbOsuoiy smeo bgi laspe ni eorniagsn ereh ubt h'tsta eon athp to hte hrtig earsnw

victor_abdullatif  edit #1: made a typo. meant to say "the patient's symptoms are NOT severe" sorry about that! +  
l0ud_minority  Yes I don't understand why the MCHC is normal that is still not what is expected. I wish NBME provided answer keys and cited their sources for their rationale. +  

submitted by sciguy(1),
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SH is ooatmauls n,ndotaim utb iixhsbte loemicntpe .eeenapctrn oS he hda no codrer fo nrapeosl ilf,mya but tlils klieyl eougterhzys.

kiWeai(pid) eTh iinlaccl evsyiert fo SH vseiar from r-oseemymptf rrcarei ot esreve soshlyeim beuesac teh oriserdd xiibehst otcimeenpl teparnenec in its .eenisrxosp

ibestalkinyo  This is what threw me off; I figured with 2 unaffected parents, it's more likely he had an autosomal recessive mutation. +1  

submitted by โˆ—strugglebus(184),
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tuoA dmo edeiass era luyalus teoeuzgyhosr (or so yeth watn su to )uessma

xxabi  How do you know is autosomal dominant? +3  
scpomp  Hereditary spherocytosis +1  
fshowon  Isnt the mean corpuscular hemoglobin concentration increased in spherocytosis? Thats what through me off. +5  
charcot_bouchard  yes, would be inc in prev NBME. But this is batshit nbme 20. U have to identify spherocytes without central pallor in PBF +4  
charcot_bouchard  yes, would be inc in prev NBME. But this is batshit nbme 20. U have to identify spherocytes without central pallor in PBF +  

submitted by โˆ—sunnyside(1),
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naC neaoyn tifsuyj hwy etyh ddi nto decbseri na dacenisre CH?CM AF 1209 asys sorhyocstpsie has hhgi CC,MH nda yhte ddi ont even heav ti on het uerpp ned fo

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