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NBME 20 Answers

nbme20/Block 3/Question#36 (45.8 difficulty score)
A 4-year-old boy has had fever, abdominal ...
Shigella sonnei🔍
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 +14 
submitted by louisville(12),
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nM-eeheuybltel idnetas fclea measr eelvedr muosnuer itpluonhrse ut(b not any )sgoisamrn. lgSaeihl is rsoslloec hnew tinsade htwi yeemtehln e;lbu E ocil asitns belu htwi leyhemnet bule ucbeesa it snemfetr .eltocsa

sympathetikey  E. coli stains green (just fyi) otherwise, perfect. +11  
chandlerbas  only E coli stains green, all other lactose fermenters stain purple/black (just fyi) otherwise, perfect. ;) FA144 bottom +3  
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: https://www.ncbi.nlm.nih.gov/pubmed/4554412 +2  



 +7 
submitted by monoloco(125),
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I ntkhi lailgeSh is het sotm eiap,prptroa sa it is llctaauy dedgerar as gyihhl aanrl.ymifmot Ys,e .E ocli nac be of eht ECCEHST/E ertvaiy, utb .E ocli cduol oals be fo teh CTEE iyreavt ro avreethw rthoe starnis it h.as rogE, .E clio mya be a,bsuielpl tbu it is not the mso't ylek.li' Blhe to seeht dnski fo st.oeuqins

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



 +7 
submitted by beeip(118),
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rAtap mrof eht neli ni AF rneeirefngc NPM eiftlirant in Sgalleih, reeht is on yaw to ntieeeirtdffa eehr benetwe it nad E. iloC. eCpha so.th

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. +  



 +7 
submitted by neonem(514),
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iSahellg eacuss na manmfyaroitl rh;rdieaa it upcdreos a noixt dan nac vdneai tussie eitcldry. In ,oiniatdd it si tsestnira to ,cdai os it sah a cirahyaelclaricstt lwo nfetcveii sdoe 0~1( sa)rosg,imn hchiw eiiaaltsfct tsi laaleforc- (noeprsopson-et-)r dasrpe eyaelscilp in neststgi eherw igeyneh yma be oodcpimsem,r csuh as in dayreac ro itlnuintsoiat u.ngiosh It can eb fatridenfieedt mfor .E Coil CH)E(E eauebcs E lCio n'soetd aehv as umch nnere-r-sotsopop edarsp adn yoln scuesa IG ademga yb eht -iikehlags xot,ni otn tdrcie nnoisaiv. he,frerTeo EEHC ut'wlond tefiltciaa as otgrsn fo a ilpiurcotnhe eser.npso

yotsubato  I assumed all the kids in the daycare had the same lunch, thus got food poisoning, thus all got EHEC. +1  



 +2 
submitted by groovygrinch(10),

Like many here, I was between E. Coli and Shigella but went with Shigella because of the daycare center history. I associate EHEC E. Coli with bad burger meat and it seemed unlikely to me that they would be whipping up burgers at the daycare (Usually everyone brings their own lunch to daycare) .




 +1 
submitted by step1soon(43),

sOn EMB Agar, isolated colonies of lactose-fermenting bacteria appear brown to blue-black in color. Escherichia coli appears as large, blue-black colonies, often with a green metallic sheen. Enterobacter spp. present as brown to blue-black, mucoid colonies with no sheen. Non-lactose-fermenting colonies, such as Shigella spp. and Salmonella spp., appear transparent and colorless.

We basically needed to correlate that → no organisms seen but only neutrophils to being a lactose non-fermentor organism. Therefore the answer is Shigella and not E coli