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NBME 21 Answers

nbme21/Block 4/Question#29

A 22-year-old man comes to the physician because of ...

Formation of destructive free radicals

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tinidazole preferred due to single dose

sweetmed  or metronidazole +  
niboonsh  what would his diagnosis be tho? +  
lostweightthxnorovirus  @niboonsh Giardia I believe. the trophozoite is pictured in the problem and has a classic "shield-like" appearance. FA 2019 pg. 155 has more information and the sketchy for it was really good! +1  
nwinkelmann  Per FA, DOC for giardia = metronidazole. MOA of metronidazole = formation of toxic free radical metabolites in the bacterial cell wall that damage DNA making it bactericidal and antiprotozoal. Metro treats = GET GAP = giardia, entamoeba, trichomonas, Gardnerella, anaerobes (below diaphragm), and H. pylori (as an alternative to amoxicillin in PCN allergy). Adverse effects = disulfiram-like reaction, HA, and metallic taste. I didn't know what Tinidazole is, and found out it is of the same drug class as Metronidazole, so makes sense why it would also be used for Giardia. For the purpose of the UMSLE 1, though, I think metronidazole would be DOC (especially because tinidazole isn't in FA). +1  




This is a picture of Giardia lamblia… caused by drinking contaminated water with cysts.

Multinucleated trophozoites

Rx -- Metronidazole -- forms toxic free radical metabolites in the bacterial cell that damage DNA = BACTERICIDAL