Antimicrobial testing: MIC and MBC: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Antimicrobial testing: MIC and MBC

Sample item

A Staphylococcus aureus isolate from a patient with endocarditis has a vancomycin MIC of 1 mg/L, which is in the susceptible range, but the minimum bactericidal concentration is 64 mg/L. The patient has persistent bacteraemia despite adequate dosing. How is this isolate best described?

AHeteroresistant, because the MIC is in the susceptible rangeBResistant, because the MBC is highCTolerant: the drug inhibits growth but does not readily kill, with an MBC to MIC ratio of 64DSynergistic with the antibiotic, because the ratio is above 4

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Tolerant: the drug inhibits growth but does not readily kill, with an MBC to MIC ratio of 64

The ratio is MBC divided by MIC, which is 64 divided by 1, giving 64. A ratio of 32 or more defines tolerance: the organism is inhibited at a normal MIC but is not killed. Tolerance can explain treatment failure in endocarditis, where bactericidal activity matters. Resistance is defined by an MIC above the breakpoint, not by the MBC alone.

Option by option

A. Heteroresistant, because the MIC is in the susceptible range
Heteroresistance is a subpopulation growing at higher drug concentrations and would be shown by population analysis, not by this ratio.
B. Resistant, because the MBC is high
Resistance is defined by a raised MIC above the breakpoint; this isolate has a susceptible MIC of 1 mg/L.
C. Tolerant: the drug inhibits growth but does not readily kill, with an MBC to MIC ratio of 64 (correct)
A ratio of 64 with a susceptible MIC defines tolerance.
D. Synergistic with the antibiotic, because the ratio is above 4
Synergy describes the interaction of two drugs and cannot be inferred from the MBC to MIC ratio of one drug.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Tolerant: the drug inhibits growth but does not readily kill, with an MBC to MIC ratio of 64. The ratio is MBC divided by MIC, which is 64 divided by 1, giving 64.

Why is "Heteroresistant, because the MIC is in the susceptible range" not the answer?

Heteroresistance is a subpopulation growing at higher drug concentrations and would be shown by population analysis, not by this ratio.

Why is "Resistant, because the MBC is high" not the answer?

Resistance is defined by a raised MIC above the breakpoint; this isolate has a susceptible MIC of 1 mg/L.

Why is "Synergistic with the antibiotic, because the ratio is above 4" not the answer?

Synergy describes the interaction of two drugs and cannot be inferred from the MBC to MIC ratio of one drug.

What concept does this question test?

It tests Antimicrobial testing: MIC and MBC in Microbiology. A ratio of 32 or more defines tolerance: the organism is inhibited at a normal MIC but is not killed. Tolerance can explain treatment failure in endocarditis, where bactericidal activity matters. Resistance is defined by an MIC above the breakpoint, not by the MBC alone.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Antimicrobial testing: MIC and MBC?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.