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NBME 20 Answers

nbme20/Block 3/Question#36 (54.8 difficulty score)
A 4-year-old boy has had fever, abdominal ...
Shigella sonnei🔍,📺

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submitted by louisville(12),
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etelMenlhbe-yu idseant felac earsm eervled eomrsuun rlsepnuthoi utb( nto yan sas)m.irong elgialhS is srsoclloe hwne idanest hwti lthemyeen eblu; E coil atisns leub hitw eenymhlte beul beascue ti srtmneef

sympathetikey  E. coli stains green (just fyi) otherwise, perfect. +14  
chandlerbas  only E coli stains green, all other lactose fermenters stain purple/black (just fyi) otherwise, perfect. ;) FA144 bottom +4  
dmotav  I think this is actually a separate concept – E. coli stains "metallic green" on eosin methylene blue agar, which is in the E. coli sketchy and first aid. However this question is referring to the methylene blue test on a fecal smear. From what I can gather, this test will be positive (indicated by presence of neutrophils) in cases of invasive diarrhea (i.e. shigella, salmonella, enterohemorrhagic E. coli). The test will be negative (no neutrophils) in cases of diarrhea caused by toxins (cholera, enterotoxigenic E. coli, giardia, viral diarrhea). So even though E. coli can present with fecal PMNs (if it's the enterohemorrhagic type), I guess that's less likely than shigella? source: +4  
srmtn  correct @dmotav, Eosin methylene blue agar is different from Methylen Blue stain which just show presence or not of leucocytes... just read this: Fecal leukocytes can be observed in those with the following conditions • Salmonella infections • Shigella infections • Ulcerative colitis • Crohn’s disease Leukocytes will be absent in fecal samples from those with Giardia, E.coli or viral infections and also in cases of food poisoning. +  

submitted by beeip(129),
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ratpA ofrm eth neil in AF eernfgincer PNM faleriittn ni lSeialgh, ehert si no wya ot ieateitfndrfe eehr tbwneee ti dna E. oiCl. eapCh tos.h

merpaperple  As best I can understand, what they are trying to get at is that "EMB stain showing neutrophils" -> "EMB stain NOT showing organisms" -> more likely Shigella than E Coli. If a special culture for an organism (e.g. EMB for E. Coli) shows no organisms but only inflammatory cells, i.e. neutrophils, that means that organism is not present. If that's the point here it's kind of a trick question, as we all would have gotten it right if they had just said "EMB showed no organisms." I guess the takeaway is that the point of a special culture is binary - you are looking for a specific organism, and it is either there or it isn't. +1  

submitted by monoloco(141),
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I iknht hlaigelS si teh otsm rtairp,popae as ti is lactauly erdaedrg sa hilghy myotmrlf.aina Ye,s E. olic nca eb of eht ESCEEHCT/ evai,yrt btu .E ocil udloc aslo be of het CEET tayeriv or eeavwhrt hoetr irstsan it goEr, E. ilco may be uall,ipseb but it is ont eth mt'os ly'kei.l lBhe ot eeths snkdi fo qtissnou.e

jcmed  This is why I picked this one because of the mucoid stools/inflammatory nature :) +  

submitted by neonem(578),
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hiagSlel ucases na lfoamiymanrt re;aiahdr it esuocdrp a ionxt adn anc adievn isuste .crlteydi In dtnidai,o ti si rsitntsae ot cdai, so it ahs a yeashtclclrctariia wol feitvncei osed (10~ sr,mnagoi)s ihwhc ieltaftscai sti lc-roaleaf -ton)srepeso-(rnop spdaer selylciape ni tigtssen wehre nhgeyei may be p,emosimdroc cuhs sa ni dacyear or isnitontutila onug.shi It cna be reftiatnedidef ofmr .E oliC )HEEC( bauesec E olCi tdne'os aehv as muhc s-opseroot-enrpn saprde dan ynlo ausecs GI dgmaae yb eht aiihks-leg nxito, tno cterdi isonainv. efe,oerThr HCEE twlnoud' lactiietaf as gtsrno of a iionucerlhtp eesrsn.op

yotsubato  I assumed all the kids in the daycare had the same lunch, thus got food poisoning, thus all got EHEC. +4  

submitted by groovygrinch(27),
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eLki yman ,eher I swa wbtneee E. Cilo dna hgliaelS tub tewn ithw agielhSl ecsaube fo eht ydcraae enrect rity.ohs I saoaietcs CEEH E. Coli hiwt dba rbgure aemt nda it dmeese ulnkyiel ot em thta tyeh loduw eb pghwpiin up gsrerub at teh ydaerac sl(layuU eroyenve nsrgbi erith onw cluhn ot a)aycedr .

submitted by step1soon(46),
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Ons BEM argA, osdletia soleoicn of rlnftges-icaeontme airbcate parpea nbwor to becblk-lua ni oorl.c cacherhisiE loci paspaer sa ,leagr cu-ableblk lcineoso, eotnf hwit a green ltmceila .hsene ronecettrbaE s.pp psenetr as rnwbo to ,aubcelblk- cmudio oslceino whti no eeshn. omnira-et-olnnNgtsfeec ,enliocos scuh as ealilhgS p.sp dan eaalllnomS p.sp, appaer stnnrraaetp dna eos.lclsro

We blasaiycl deeden to areoltrec hatt → no sgnmirsoa nsee tub lnoy intphresoul ot eginb a caloest nmntoefrone-r anrsimog. erfTeeohr the srwnea si alhieSgl and ton E cilo

submitted by feralbaskin(1),

Shigella is the most likely causal organism over E. coli due to the vignette specifically stating the patient has had "bloody MUCOID stools with tenesmus."

Page 144 FA 2019 GI manifestations: Fever, crampy abdominal pain -> tenesmus, blood mucoid stools (bacillary dysentery).

Please correct me if I'm wrong, but I don't remember Sketchy mentioning bloody mucoid stools for E. coli and that detail doesn't seem to appear in FA.

meja2  Thank you! I didn't catch the "bloody mucoid stools" reference, and yes you're right- E. coli wouldn't have this presentation! +  

submitted by beetbox(2),

Can someone help me out? I chose E.Coli too, and I'm too thick to understand the comments here. The question said fecal smear not special agar culture dishes...why are we talking about stain colors? and why would methylene blue stained fecal smear show no organism? I thought methylene blue stains just about any cells that has DNA/RNA. If the question is trying to tell us that there were NO ORGANISMS seen in the smear, why would Shigella not be seen? Is it because Shigella invades cells and considered more intracellular than E.Coli?

topgunber  i think E. Coli would be visible on a methylene blue stain (green stain), i'm assuming shigella is negative. virio parahemolyticus causes bloody diarrhea unlike cholera but im guessing its wrong because its from seafood. Shigella and c. jejuni are common in the states (same with e coli). Because only neutrophils were stained we have to assume that its not e coli. +