Errors in First Aid for the USMLE (2007): Microbiology

As always, comments are welcome. I’m sorry to say that I did not do a good job reviewing the section on viruses as this is my weakest subject.

Microbiology

  1. P.137, Bugs with exotoxins
    1. Bordetella pertussis does not stimulate adenylate cyclase, it instead inhibits GTPase. This differentiates its action from that of cholera toxin and the LT toxin of E.coli, whose actions stimulate adenylate cyclase.
  2. P.140, Intracellular bugs
    1. For facultative intracellular, I offer the following:

i. My Liege, Your Niece Lists Frank, Bruce and Sam.

ii. Mycobacterium, Leigonella, Yersinia, Neisseria, Listeria, Francisella, Brucella, Salmonella.

  1. P.144, Lactose-fermenting enteric bacteria
    1. After including Serratia, change the mnemonic from “lactose is KEE” to:

i. “Test lactose with MacConKEE’S”.

ii. Citrobacter, Klebsiella, E.coli, Enterobacter, Serratia.

  1. P.145, Bugs causing diarrhea
    1. O157:H7 should refer to Enterohemorrhagic E.coli (EHEC), not Enteroinvasive E.coli.
  2. P.150
    1. The heading “Microbiology-Mycology” is on the wrong page, and should be on P.151.
  3. P.152, Pneumocystis carinii
    1. This microbe is now referred to as Pneumocystis jeroveci.
  4. P.154, Medically important helminths
    1. There should be some mention that Schistosomiasis can cause granulomas in the bladder and has a role in Squamous cell carcinoma of the bladder.
  5. P.163, HIV diagnosis
    1. A test with high sensitivity has low false-positives, not high. A sensitive test with high false-positives indicates that there is low prevalence of the tested disease in the population. It is more appropriate to use NPV for this type of statement.
    2. A test with high specificity has low false-negatives, not high. A specific test with high false-negatives indicates that there is a low prevalence of the tested disease in the population. It is more appropriate to use PPV for this type of statement.

i. You may not think that these distinctions are important, but they are. Sensitivity and specificity are qualities of a test and do not change depending on the population tested, but a test conducted in Africa (where prevalence of HIV is high) versus the same test conducted in the US (where the prevalence is low) will have different PPVs and NPVs, i.e., different numbers of false-positive and false-negative results.

  1. P.164, Prions
    1. Fatal Familial Insomnia should be included in this list of Prion diseases.
  2. P.169, Bactericidal antibiotics
    1. I think that Rifampin, daptomycin, the combination treatment SMX/TMP and the polymyxins should be included in the list of cidal drugs
  3. P.169, Methicillin….
    1. “Don’t need MeNDing: Methicillin, Nafcillin, Dicloxacillin”
  4. P.170, Cephalosporins
    1. The MTT group responsible for the disulfiram-like reaction is only found in 2nd generation cephalosporins cefotetan and cefamandole. I think it’s worth changing to “(in 2nd generation cephalosporins with a methylthiotetrazole group, e.g. cefamandole and cefotetan)”.
  5. P.172, Macrolides
    1. I think it’s worth mentioning that Erythromycin is a potent inhibitor of P450, that Azithromycin is used in prophylaxis of MAC, and that their clinical use is for atypical pneumonias.
  6. P.172, Clindamycin
    1. Lincomycin is listed on P.171 as one of the 50S inhibitors, but it is not mentioned that this drug belongs to the same family as Clindamycin. I think this should be changed to “Clindamycin, Lincomycin
  7. P.173, Trimethoprim
    1. I think that the following grouping is interesting:

i. Methotrexate – inhibits human Dihydrofolate reductase

ii. Trimethoprim – inhibits microbial Dihydrofolate reductase

iii. Pyrimethamine – inhibits parasitic Dihydrofolate reductase

  1. P.176, Antifungal therapy
    1. The antimicrobials were listed as being either cidal or static, but this is not done for the antifungal drugs. I think this should be included with each description.

i. Polyenes (Amp B and Nystatin) – cidal

ii. Azoles – static

iii. Flucytosine – cidal

iv. Caspofungin – cidal

v. Terbinafine – static

vi. Griseofulvin – static

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21 Responses to Errors in First Aid for the USMLE (2007): Microbiology

  1. Hey Topher, it;s been a while. Hope your exams are going well – exam season has started here so it’s my turn to descend into the pit (though I doubt I’ll escape with as much of my sanity intact as you have).

    Anyhoo, just dropping a note to say that I’ve tagged you with the Thinking Bloggers Award. Your posts really do make me think, and so yours was one of the first blogs that came to mind when I got the award. You are of course not obligated to participate, but do show off the award – you deserve it.

    Keep it up! I’ll be back.

  2. Ben says:

    pg. 158 in the virology section

    MMR, with the R [Reubella] being bolded, is indicated as being an egg based vaccine.

    However, measles and mumps are the egg based vaccines, while Reubella is from lung fibroblasts.

    Source. http://en.wikipedia.org/wiki/MMR_vaccine

  3. Elian says:

    Hi, actually page 163 the way that it was written is correct.

    A sensitive test means the threshold is low so you will have high false positive rate but low false negative rate. If the sensitive test is positive it means that you might have the disease and need to check with a more specific test bit you shouldnt be too worried because the sensitive test has HIGH false positives

    The specific test has a higher threshold so it has much lower false positives and high false negative. Thats why you dont doo screening with a specific test because you dont want to miss anybody who has the disease but this test has a HIGH false negative rate.

    thank you

  4. Elian says:

    Another way to look at it is that Sensitive test is Rule Out test so if the sensitive test comes back negative you can go home happy because it really means that you dont have the HIV (low false negatives)

    Similarly the Specific test is a Rule In test so if it comes back positive it really means that you have HIV. (low false positives)

    Everything else looks good, keep up the good work :)

  5. Daniel says:

    Thanks for the hard work!

    According to Wikipedia, cefmenoxime (3rd gen cephalosporin) also has a MTT group, so I don’t think the disulfiram-like reactions are limited to 2nd gen cephalosporins.

  6. Nelson says:

    I think the entry for bugs with exotoxins (p137) could be clarified: B. pertussis ADP ribosylates Gi (inhibitory G protein), turning it off, which leads to an increase in cAMP. You could change the 1st word of the sentence as it is written in FA2007 from “Stimulates AC” to “Disinhibits AC”.

  7. Daniel says:

    Again, thanks for the work and congrats on the high score! Good karma comes back around.

    Serratia is a NON-lactose fermenter, but it’s a “wannabe” in that it produces pink colonies on MacConkey’s agar anyway.

  8. Nelson says:

    Just to confirm what Danial said, my Katzung & Trevor pharm book lists 3 cephalosporins with an MTT group which can cause disulfiram-like reactions: cefamandole and cefotetan (2nd generation) and cefoperazone (3rd gen).

    And, to contradict another post of Daniel’s, my Rapid Review micro 2nd edition does list Serratia as a lactose fermenter.

  9. Daniel says:

    After looking it up some more, even though our school taught us that it was a non-lactose-fermenter, Serratia seems to be officially a “slow” lactose fermenter. Don’t believe everything you’re taught, I guess?

  10. Holly says:

    PCP appears also on page 139, and should be changed to P. jiroveci. It’s also pretty shady that on first reference they just called it “PCP” without an explanation.

  11. Holly says:

    p.163, HIV diagnosis.
    The paragraph is wrong, but not in the way you described. The ELISA is sensitive, which means it has a low false-negative rate (it calls most people with the disease positive and only lets a few slip past thinking they’re negative). This is good because for the first test you want to call as many people positive as possible. Then you confirm them all with the Western blot, which is highly specific, which means it has a low false-positive rate, so only those who truly have the disease will still test positive. (Sensitivity and specificity are calculated down the columns of the 2×2 table, so sensitivity is associated with true positives and false negatives, not with false-positives.)

  12. Holly says:

    p.171, I changed the 50S mnemonic to: chloramphenicol, erythromycin, lincomycin/clindamycin, Linezolid. I did this because as you noted, lincomycin/clindamycin are in the same class, and they don’t mention linezolid, which is important as a 50S inhibitor used to treat MRSA.

  13. mike says:

    Pg. 137…. Bordetella Pertussis actually does stimulate the Adenylate cyclase toxin. The pertussis A subunit transfers the ADP ribosyl moiety of NAD to the membrane-bound regulatory protein Gi that normally inhibits the eukaryotic adenylate cyclase. Hence, the Gi protein is inactivated and cannot perform its normal function to inhibit adenylate cyclase.

    Be Careful what you post !!!

  14. mike says:

    Pg 137. Another point to make is that the Mechanisms of all 3 V. cholerae, E. coli, & Bordetella pertussis are essentially similar. This is the main reason why they are classified under the same heading of “ADP Ribosylating A-B toxins”.

    You differentiate them not by mechanism but instead by history.

  15. mike says:

    Pg 170 Cephalosporin toxicity …… MTT group is actually present in both 2nd & 3rd generation cephalosporins. Cefamandole is just cited as an example.

  16. Nick says:

    # Holly Says:
    May 2nd, 2007 at 12:39 am

    PCP appears also on page 139, and should be changed to P. jiroveci. It’s also pretty shady that on first reference they just called it “PCP” without an explanation.

    PCP = P. Carinii Pneumonia

  17. Liam says:

    want to share a good/ridiculous mnemonic for the +ss RNA viruses… my buddy came up with this one:

    Flava-flave is having a Retro Toga&Corona party in Pico Cali

    Flavivirus
    Retreovirues
    Togavirus
    Coronavirus
    Picornavirus (no envelope)
    Calicivirus (no envelope) “you don’t need mail an Envelope to RSVP”

  18. Jakub says:

    B pertussis stimulates AC via inhibition of Gi.
    It does inhibit the GTPase activity of the Gi protein, so that’s probably where the confusion arose.

  19. Sureshot says:

    Page 174: referring to anti-TB drugs (streptomycin, pyrazinamide, isoniazid, rifampin, ethambutol)

    “All are hepatotoxic.” –> Not exactly true, because ethambutol’s main side effect is optic neuritis with associated loss of red/green color vision. If anything, hepatotoxicity might be like the “17th side effect” or something, but it certainly isn’t as common as it is for the other anti-TB drugs.

  20. Adam says:

    Page 149: Chlamydia trachomatis serotypes

    Why do they even mention the positive Frei test for lymphogranuloma venereum? It is a hypersensitivity reaction to an intradermal antigen common to ALL chlamydia serotypes, so it isn’t specific for LGV at all. Not to mention the fact that the commercial test was discontinued in 1974!

  21. Benji says:

    On p. 145, shouldn’t C. diff be listed as watery diarrhea? it doesn’t usually cause bloody diarrhea.

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