Antimicrobial susceptibility: Etest: 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 susceptibility: Etest

Sample item

On an Etest plate, a lawn of an isolate shows an elliptical zone of inhibition whose edge meets the antibiotic gradient strip at the 0.5 mark on the scale. What does this reading represent?

AThe post-antibiotic effect lasts 0.5 hoursBThe zone diameter is 0.5 mmCThe minimum inhibitory concentration is 0.5 micrograms per millilitreDThe minimum bactericidal concentration is 0.5 micrograms per millilitre

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

The answer and the reasoning

C. The minimum inhibitory concentration is 0.5 micrograms per millilitre

The Etest strip carries a predefined exponential gradient of antibiotic. After incubation, the MIC is read at the point where the edge of the inhibition ellipse intersects the strip. The value is then interpreted using breakpoints.

Option by option

A. The post-antibiotic effect lasts 0.5 hours
Post-antibiotic effect is a pharmacodynamic property and cannot be read from a strip.
B. The zone diameter is 0.5 mm
Zone diameters are measured in millimetres in disc diffusion, not read from a gradient strip scale.
C. The minimum inhibitory concentration is 0.5 micrograms per millilitre (correct)
Correct. The intersection of the ellipse with the strip gives the MIC.
D. The minimum bactericidal concentration is 0.5 micrograms per millilitre
The minimum bactericidal concentration requires subculture from clear tubes and is not read from a strip.

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, The minimum inhibitory concentration is 0.5 micrograms per millilitre. The Etest strip carries a predefined exponential gradient of antibiotic.

Why is "The post-antibiotic effect lasts 0.5 hours" not the answer?

Post-antibiotic effect is a pharmacodynamic property and cannot be read from a strip.

Why is "The zone diameter is 0.5 mm" not the answer?

Zone diameters are measured in millimetres in disc diffusion, not read from a gradient strip scale.

Why is "The minimum bactericidal concentration is 0.5 micrograms per millilit…" not the answer?

The minimum bactericidal concentration requires subculture from clear tubes and is not read from a strip.

What concept does this question test?

It tests Antimicrobial susceptibility: Etest in Microbiology. After incubation, the MIC is read at the point where the edge of the inhibition ellipse intersects the strip. The value is then interpreted using breakpoints.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 susceptibility: Etest?

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.