Antimicrobial testing: fractional inhibitory concentration: 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: fractional inhibitory concentration

Sample item

In a checkerboard assay, the MIC of drug A alone is 8 mg/L and of drug B alone is 16 mg/L. In the best combination well, the MICs are 2 mg/L of A and 4 mg/L of B. What is the fractional inhibitory concentration index and the interpretation?

A1.0, indicating antagonismB2.0, indicating synergyC0.25, indicating indifferenceD0.5, indicating synergy

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

The answer and the reasoning

D. 0.5, indicating synergy

FIC index equals the sum of each drug's FIC, where FIC is the MIC in combination divided by the MIC alone. For drug A it is 2 divided by 8, which is 0.25, and for drug B it is 4 divided by 16, which is 0.25. The sum is 0.5, and an index of 0.5 or less is interpreted as synergy. Values above 0.5 up to 4 mean indifference, and above 4 means antagonism.

Option by option

A. 1.0, indicating antagonism
An index of 1.0 would be indifference, and antagonism requires an index above 4.
B. 2.0, indicating synergy
An index of 2.0 would be indifference, not synergy, and the calculation here does not give 2.0.
C. 0.25, indicating indifference
0.25 is a single drug's FIC rather than the index, and low values indicate synergy, not indifference.
D. 0.5, indicating synergy (correct)
The sum 0.25 plus 0.25 is 0.5, which meets the threshold for synergy.

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 D, 0.5, indicating synergy. FIC index equals the sum of each drug's FIC, where FIC is the MIC in combination divided by the MIC alone.

Why is "1.0, indicating antagonism" not the answer?

An index of 1.0 would be indifference, and antagonism requires an index above 4.

Why is "2.0, indicating synergy" not the answer?

An index of 2.0 would be indifference, not synergy, and the calculation here does not give 2.0.

Why is "0.25, indicating indifference" not the answer?

0.25 is a single drug's FIC rather than the index, and low values indicate synergy, not indifference.

What concept does this question test?

It tests Antimicrobial testing: fractional inhibitory concentration in Microbiology. For drug A it is 2 divided by 8, which is 0.25, and for drug B it is 4 divided by 16, which is 0.25. The sum is 0.5, and an index of 0.5 or less is interpreted as synergy. Values above 0.5 up to 4 mean indifference, and above 4 means antagonism.

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: fractional inhibitory concentration?

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.