FREE AP-MICRO Antimicrobial Susceptibility Testing Questions and Answers

Question 1

A Staphylococcus aureus isolate from a wound culture shows resistance to erythromycin but appears susceptible to clindamycin on an automated susceptibility testing panel.
Which of the following tests should be performed to detect potential inducible resistance?

Accepted Answer

D-test

Answer

Cefoxitin disk screen

Answer

Modified Hodge test

Answer

Oxacillin salt agar screen

Question 2

When performing a Kirby-Bauer disk diffusion test, a technologist uses a Mueller-Hinton agar plate with a depth of 6 mm, which is thicker than the CLSI recommended standard of 4 mm. What effect will this have on the zone sizes?

Accepted Answer

Zone sizes will be smaller, potentially leading to false-resistant results.

Answer

Zone sizes will be larger, potentially leading to false-susceptible results.

Answer

Zone sizes will be unaffected by the agar depth.

Answer

Zone sizes will be inconsistent and the test will be invalid regardless of the results.

Question 3

A laboratory is performing the CLSI-recommended phenotypic confirmatory test for Extended-Spectrum Beta-Lactamase (ESBL) production in a Klebsiella pneumoniae isolate. A positive test is indicated by which of the following results?

Accepted Answer

A ≥ 5 mm increase in the zone diameter for ceftazidime tested with clavulanic acid versus ceftazidime tested alone.

Answer

A zone of inhibition around a meropenem disk that is ≤ 21 mm.

Answer

Resistance to both cefotaxime and ceftazidime.

Answer

Growth on Mueller-Hinton agar containing a 4 µg/mL concentration of cefoxitin.

Question 4

Which of the following sets of organisms are the standard, non-fastidious quality control strains recommended by CLSI for routine disk diffusion and MIC testing?

Accepted Answer

Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853

Answer

Enterococcus faecalis ATCC 51299, Klebsiella pneumoniae ATCC 700603, Escherichia coli ATCC 35218

Answer

Staphylococcus aureus ATCC 43300, Haemophilus influenzae ATCC 49247, Streptococcus pneumoniae ATCC 49619

Answer

Candida albicans ATCC 90028, Aspergillus fumigatus ATCC 204305, Cryptococcus neoformans ATCC 90112

Question 5

A clinical laboratory reports a vancomycin MIC of 2 µg/mL for a Staphylococcus aureus isolate from a blood culture. The CLSI interpretive breakpoint for susceptible is ≤2 µg/mL. How should this result be interpreted?

Accepted Answer

Susceptible

Answer

Resistant

Answer

Intermediate

Answer

Invalid, test must be repeated

Question 6

Antimicrobial susceptibility testing for Streptococcus pneumoniae requires specific modifications to the standard Kirby-Bauer procedure. Which of the following is a required modification?

Accepted Answer

Incubation in an atmosphere of 5-7% CO2 on Mueller-Hinton agar with 5% sheep blood.

Answer

Incubation in an anaerobic environment for 24-48 hours.

Answer

Use of Haemophilus Test Medium (HTM) agar.

Answer

Inoculation of a 1.0 McFarland standard suspension.

A Staphylococcus aureus isolate from a wound culture shows resistance to erythromycin but appears susceptible to clindamycin on an automated susceptibility testing panel.Which of the following tests should be performed to detect potential inducible resistance?

A Staphylococcus aureus isolate from a wound culture shows resistance to erythromycin but appears susceptible to clindamycin on an automated susceptibility testing panel.
Which of the following tests should be performed to detect potential inducible resistance?