Antimicrobial Resistance Mechanisms and Susceptibility Testing (ESBL, MRSA, VRE, CRE, MIC/MBC, E-test) MCQs

Microbiology · 53 free questions with answers & explanations.

1. A clinical microbiology lab receives a urine isolate of Klebsiella pneumoniae from a catheterized ICU patient. Disk diffusion shows resistance to ceftriaxone and ceftazidime but the organism tests susceptible to cefoxitin and imipenem. The phenotypic confirmatory test for ESBL production should be:
2. A 60-year-old patient with a hip prosthesis develops a peri-prosthetic joint infection (PJI). Cultures from intraoperative tissue samples grow coagulase-negative Staphylococcus (CoNS) resistant to oxacillin (MIC >4 μg/mL). The molecular basis of oxacillin resistance in CoNS is:
3. A patient with a KPC-producing Klebsiella pneumoniae bloodstream infection is being considered for ceftazidime-avibactam therapy. Avibactam is a novel beta-lactamase inhibitor. Which class of carbapenemases does avibactam inhibit, and which does it NOT inhibit?
4. E-test (Epsilometer test) is used to determine MIC of an antibiotic. A gram-positive coccus is tested for vancomycin susceptibility. The E-test strip shows the MIC ellipse intersecting the strip at 3 μg/mL. According to CLSI breakpoints, this isolate would be classified as:
5. A patient with hospital-acquired pneumonia grows pan-drug-resistant (PDR) Acinetobacter baumannii from BAL culture. The only remaining option is colistin (polymyxin E). The mechanism of colistin resistance in Acinetobacter most commonly involves:
6. A blood culture isolate is identified as MRSA. The mechanism of methicillin resistance is:
7. An E-test strip for vancomycin against a Staphylococcus aureus isolate shows an elliptical inhibition zone intersecting the strip at MIC 6 μg/mL. According to CLSI breakpoints, this isolate is classified as:
8. A Klebsiella pneumoniae isolate from an ICU patient with pneumonia shows meropenem MIC of 32 μg/mL. Modified Carbapenem Inactivation Method (mCIM) is positive. KPC is detected by molecular testing. What is the treatment of choice?
9. A Pseudomonas aeruginosa isolate is susceptible to piperacillin-tazobactam alone but resistant to imipenem. OXA-48 is detected by PCR; however metallo-beta-lactamase (MBL) screen with EDTA disk synergy is negative. The resistance mechanism is:
10. A blood culture isolate of Klebsiella pneumoniae from an ICU patient shows resistance to all carbapenems (MIC meropenem >8 µg/mL). Carbapenemase testing by modified carbapenem inactivation method (mCIM) is positive. The EDTA-modified CIM (eCIM) is POSITIVE (synergy with EDTA). What class of carbapenemase is most likely present, and what is the therapeutic implication?
11. The E-test (epsilometer test) is used to determine MIC of vancomycin against a Staphylococcus aureus isolate. The MIC read is 4 µg/mL. According to CLSI breakpoints for S. aureus, how is this isolate classified, and what is the clinical concern?
12. Enterococcus faecium is isolated from a blood culture. Disc diffusion shows a vancomycin zone of 8 mm (resistant). PCR confirms vanA gene. The mechanism of vanA-mediated resistance is:
13. A urine culture grows Klebsiella pneumoniae. The Kirby-Bauer disc diffusion shows reduced inhibition zones for ceftazidime and cefotaxime compared to controls. Double disc synergy test (DDST) using clavulanate-augmented discs shows ≥5 mm increase in zone diameter. Which resistance mechanism is confirmed and which antibiotic is definitively active?
14. A blood culture grows MRSA. The isolate's vancomycin MIC by E-test is 1.5 mg/L (susceptibility breakpoint: ≤2 mg/L). Clinical response is poor despite adequate vancomycin trough levels of 15-20 mg/L. Which concept best explains this treatment failure?
15. A carbapenem-resistant Klebsiella pneumoniae (CRKP) isolate grows on blood agar. Modified carbapenem inactivation method (mCIM) is positive; EDTA mCIM (eCIM) is also positive. This pattern (both mCIM positive and eCIM positive) is most consistent with:
16. For an organism with MIC of 4 mg/L and MBC of 64 mg/L for drug X, the drug is classified as:
17. A Klebsiella pneumoniae blood culture isolate shows: MIC to imipenem >32 µg/mL, MIC to meropenem >16 µg/mL, and synergy on double-disc diffusion with EDTA. Modified carbapenem inactivation method (mCIM) is positive. The most likely resistance mechanism is:
18. An MRSA strain is confirmed by the cefoxitin disc diffusion method (inhibition zone ≤21 mm). The molecular basis of methicillin resistance in MRSA is:
19. An E-test strip for cefotaxime placed on an agar plate containing an E. coli isolate shows an elliptical inhibition zone intersecting the strip at 0.5 µg/mL. If the EUCAST/CLSI breakpoint for susceptibility of cefotaxime for E. coli is ≤1 µg/mL, and resistant if ≥4 µg/mL, how would this isolate be categorized?
20. Vancomycin-resistant Enterococcus (VRE) most commonly carries which resistance gene, and what structural alteration does it produce?
21. For detecting ESBL-producing E. coli in a clinical isolate, the CLSI-recommended phenotypic confirmatory test is:
22. A urine culture isolate of E. coli shows resistance to ceftriaxone, cefotaxime, and ceftazidime but is susceptible to cefoxitin and imipenem. The double-disc synergy test (DDST) using cefotaxime and cefotaxime+clavulanate discs shows a ≥ 5 mm increase in zone size with the combination. This confirms:
23. Vancomycin-resistant enterococci (VRE) carrying the vanA gene cluster differ from vanB-carrying VRE primarily by:
24. A blood culture isolate of Klebsiella pneumoniae shows an imipenem MIC of 4 µg/mL and a meropenem MIC of 8 µg/mL by E-test. The Modified Carbapenem Inactivation Method (mCIM) test is positive. The most likely resistance mechanism is:
25. The minimum biofilm eradication concentration (MBEC) of an antibiotic is typically how many folds higher than the planktonic MBC?
26. Which statement accurately describes the mechanism of tigecycline resistance in Acinetobacter baumannii?
27. An E. coli isolate from a UTI shows resistance to cefotaxime and ceftazidime but susceptibility to cefoxitin and carbapenems. The ESBL phenotype is confirmed by the combination disc test showing ≥5 mm increase in inhibition zone with cefotaxime + clavulanic acid versus cefotaxime alone. Which enzyme class is most likely responsible?
28. An Enterococcus faecium isolate shows high-level resistance to vancomycin (MIC >256 µg/mL) and teicoplanin (MIC >128 µg/mL). Genotyping reveals vanA gene. What is the mechanism and what is the treatment option?
29. An E-test strip for meropenem placed on an MHA plate with a Klebsiella pneumoniae isolate shows an elliptical inhibition zone touching the strip at an MIC value of 8 µg/mL. What is the interpretation per CLSI 2023 breakpoints for K. pneumoniae, and what does this suggest?
30. In the broth microdilution method, a series of antibiotic concentrations is prepared and inoculated with a standardised bacterial suspension (0.5 McFarland). After 18–24 hours incubation, wells 1–4 show turbidity and wells 5–12 show no turbidity. The lowest concentration in a clear well is the MIC. The contents of the clear wells are subcultured to agar plates — wells 5 and 6 show no growth but wells 7–12 show growth. What is the MBC in this experiment?
31. A clinical microbiology lab reports a Klebsiella pneumoniae blood culture isolate as carbapenem resistant. Meropenem MIC is 4 µg/mL (resistant). Modified carbapenem inactivation method (mCIM) is positive. Phenotypic testing with EDTA-mCIM is also positive. Which enzyme class is most likely responsible?
32. In disk diffusion susceptibility testing (Kirby-Bauer), an MRSA isolate gives a cefoxitin disk zone of 18 mm (CLSI breakpoint: susceptible ≥22 mm). To confirm the mecA gene-mediated resistance mechanism, which molecular test is most specific?
33. An E-test strip for vancomycin is placed on a Mueller-Hinton agar plate inoculated with a Staphylococcus aureus isolate. The ellipse of inhibition intersects the strip at 3 µg/mL. According to CLSI breakpoints, how should this isolate be classified?
34. A urinary isolate of Escherichia coli produces a zone of inhibition of 18 mm with ceftriaxone (breakpoint ≥26 mm susceptible by CLSI). The double disc synergy test (DDST) shows a 'keyhole' enhancement of the zone between amoxicillin-clavulanate and ceftazidime discs. What is the clinical implication for treatment choice?
35. A clinical laboratory tests a Klebsiella pneumoniae isolate from a catheter-associated urinary tract infection. The disk diffusion shows resistance to ertapenem but susceptibility to imipenem. Phenotypic testing with meropenem + EDTA reveals synergy. Which mechanism best explains this pattern?
36. A blood culture isolate of Enterococcus faecium shows a vancomycin MIC of 128 mg/L and teicoplanin MIC of 256 mg/L. The van gene responsible encodes a ligase that produces D-Ala-D-Lac instead of D-Ala-D-Ala. This is consistent with:
37. For an E. coli isolate with an ESBL, the CLSI recommends that susceptibility reports for cephalosporins should be:
38. The E-test (epsilometer test) for antimicrobial susceptibility differs from disk diffusion primarily in that it:
39. A Staphylococcus aureus isolate from a wound infection has an oxacillin MIC of 8 mg/L and the cefoxitin screen disk shows a zone of 18 mm. This isolate is best classified as:
40. Which of the following describes the role of the Modified Carbapenem Inactivation Method (mCIM) in laboratory diagnostics?
41. A 62-year-old diabetic patient develops a hospital-acquired pneumonia on day 8 of ICU admission. Bronchoalveolar lavage grows Klebsiella pneumoniae. Disk diffusion shows resistance to all third-generation cephalosporins, but the isolate is initially reported sensitive to carbapenems. The combined disk test with clavulanate shows ≥5 mm increase in zone diameter. What resistance mechanism does this isolate carry?
42. During routine surveillance in a medical ICU, a rectal swab from an asymptomatic patient grows Enterococcus faecium with MIC for vancomycin of 256 µg/mL and MIC for teicoplanin of 0.5 µg/mL. What VRE phenotype does this represent?
43. A burn wound isolate of Pseudomonas aeruginosa is tested by the E-test method. The MIC for meropenem reads 8 µg/mL (EUCAST breakpoint: sensitive ≤2, resistant ≥8). The modified carbapenem inactivation method (mCIM) test is positive (turbid broth). The Carba NP test is also positive and turns yellow rapidly. Which carbapenemase class is MOST likely present?
44. A nasal swab from a healthcare worker screens positive for MRSA. The isolate is confirmed by a cefoxitin disk diffusion (zone ≤21 mm). Which gene encodes the altered penicillin-binding protein responsible for methicillin resistance in Staphylococcus aureus?
45. For a CRE (carbapenem-resistant Enterobacteriaceae) isolate, the mCIM test is performed. The test is considered positive (carbapenemase-producing) when the zone of inhibition around the imipenem disk is:
46. A clinical microbiologist notices that an E. coli isolate from urine is resistant to trimethoprim-sulfamethoxazole, ciprofloxacin, and gentamicin, but susceptible to nitrofurantoin and fosfomycin. Genotypic testing reveals a qnrS gene. What is the mechanism of quinolone resistance conferred by qnr genes?
47. The minimum bactericidal concentration (MBC) of an antibiotic is defined as the lowest concentration that kills what percentage of the original bacterial inoculum?
48. A Klebsiella pneumoniae isolate from a urine culture is resistant to all cephalosporins but susceptible to piperacillin-tazobactam and carbapenems. Phenotypic confirmation testing using the combined disc method with clavulanic acid shows a ≥5 mm increase in zone diameter. Which resistance mechanism is most likely?
49. For a bloodstream isolate, the MIC of vancomycin is 2 mg/L (within susceptible range) but the MBC is >32 mg/L. The ratio MBC/MIC exceeds 32. This phenomenon is termed:
50. A carbapenem-resistant Enterobacteriaceae (CRE) isolate is positive by the modified carbapenem inactivation method (mCIM) but negative by the EDTA-mCIM (eCIM). Which carbapenemase class is most consistent with this result?
51. MRSA is differentiated from methicillin-susceptible S. aureus (MSSA) by the presence of a specific gene encoding a modified penicillin-binding protein. Which PBP is responsible for MRSA resistance?
52. Vancomycin-resistant Enterococcus (VRE) with vanA genotype shows which pattern of resistance?
53. An E-test strip for ciprofloxacin is placed on a Mueller-Hinton agar plate inoculated with a urinary E. coli isolate. The elliptical inhibition zone intersects the strip at a value of 0.5 mg/L. According to CLSI breakpoints (susceptible ≤0.25 mg/L, resistant ≥1 mg/L), how should this isolate be reported?