Screening: MRSA/MSSA, VRE, ESBL/CRE

6 ASCP MLS practice questions on Screening: MRSA/MSSA, VRE, ESBL/CRE, part of Microbiology. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   The preferred specimen for screening an adult for MRSA colonization is:

   1. AA rectal swab
   2. BA urine specimen
   3. CA posterior pharyngeal swab
   4. DAn anterior nares swab

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   Answer: D · An anterior nares swab

   S. aureus colonizes the anterior nares, so a nasal swab is the standard surveillance specimen, sometimes with a wound, axilla or groin swab added. Swabs are plated on chromogenic MRSA agar or tested by PCR. Rectal and perirectal swabs are used for VRE and carbapenem-resistant Enterobacterales, which colonize the gut. A positive screen shows colonization, not infection.

   • A: Rectal swabs are used to screen for VRE and CRE.
   • B: Urine is used to diagnose infection, not to screen for carriage.
   • C: A pharyngeal swab is not the standard surveillance specimen for MRSA.

2. Question 2 · Medium

   A rectal swab screen from an intensive care patient is positive for carbapenem-resistant Enterobacterales. The most appropriate action is:

   1. ARepeat the screen in a month before acting
   2. BPlace the patient on contact precautions and alert infection control
   3. CStart carbapenem therapy at once
   4. DReport the finding as a probable contaminant

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   Answer: B · Place the patient on contact precautions and alert infection control

   A positive CRE screen shows gut colonization and triggers contact precautions, cohorting and often screening of other patients on the unit. It is not in itself a reason to treat, because colonization is not infection. The isolate is kept so that the carbapenemase can be characterized, since KPC, NDM and OXA-48 differ in treatment and in how they spread. Rapid notification is the whole point of surveillance screening.

   • A: Delay allows the organism to spread on the unit.
   • C: Colonization without infection is not treated with antibiotics.
   • D: Growth on a selective screening medium is a real finding, not a contaminant.

3. Question 3 · Easy

   Which agent is added to chromogenic MRSA screening agar to select resistant strains?

   1. AVancomycin
   2. BColistin
   3. CCefoxitin
   4. DNalidixic acid

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   Answer: C · Cefoxitin

   Chromogenic MRSA agars contain cefoxitin or another oxacillin-class agent, so only methicillin-resistant staphylococci grow, while a chromogenic substrate colors S. aureus colonies. Cefoxitin is used because it induces mecA expression well. Agar containing vancomycin screens for VRE instead. A presumptive colony is confirmed by identification with a cefoxitin disk or a PBP2a test.

   • A: Vancomycin is the selective agent in agar used to screen for VRE.
   • B: Colistin, with nalidixic acid, selects gram-positive organisms broadly rather than resistant staphylococci.
   • D: Nalidixic acid is used in media for other purposes, such as Campylobacter isolation.

4. Question 4 · Hard

   A hemodialysis patient has Staphylococcus aureus bacteremia. The cefoxitin disk zone is 19 mm, with susceptible set at 22 mm or more. The report should state:

   1. ACefoxitin resistant but oxacillin susceptible
   2. BVancomycin resistance should be presumed
   3. COxacillin resistant, with beta-lactams resistant
   4. DOxacillin susceptible, with nafcillin as the drug of choice

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   Answer: C · Oxacillin resistant, with beta-lactams resistant

   Cefoxitin is a surrogate for mecA-mediated resistance: a zone of 21 mm or less for S. aureus means the isolate is oxacillin (methicillin) resistant. Penicillins, cephalosporins other than the anti-MRSA agents, and beta-lactam with inhibitor combinations are then all reported resistant, whatever the in vitro result shows. Cefoxitin itself is not offered as a treatment option. Vancomycin is tested separately and is usually active.

   • A: Cefoxitin results are reported as oxacillin results; the two are not separated.
   • B: Methicillin resistance says nothing about vancomycin, which is tested separately.
   • D: A 19 mm zone is below the susceptible breakpoint, so the isolate is resistant.

5. Question 5 · Medium

   What is the main purpose of admission screening for MRSA in a high-risk unit?

   1. ATo find carriers needing contact precautions
   2. BTo choose empiric antibiotics for every admission
   3. CTo audit how well the unit is cleaned
   4. DTo diagnose staphylococcal bacteremia early

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   Answer: A · To find carriers needing contact precautions

   Active surveillance finds the reservoir of silent carriers so that contact precautions, nasal mupirocin and chlorhexidine washing can be used. In high-risk units this reduces transmission and surgical site infection. A screen is not a diagnostic culture; infection is diagnosed from the clinical specimen. Screening results are reported as colonization, with no susceptibility testing beyond the methicillin result.

   • B: Empiric therapy is a clinical decision and is not driven by surveillance swabs.
   • C: Cleaning is audited by environmental sampling and inspection.
   • D: Bacteremia is diagnosed by blood culture, not by a nasal swab.

6. Question 6 · Medium

   How is methicillin resistance in Staphylococcus aureus best detected?

   1. AA penicillin disk and a beta-lactamase test
   2. BA vancomycin agar screen plate
   3. CCefoxitin disk, or a mecA or PBP2a test
   4. DAn optochin disk on blood agar

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