General microbiology and sterilisation: aminoglycoside-modifying enzymes: 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 · General microbiology and sterilisation: aminoglycoside-modifying enzymes

Sample item

A plasmid in a Klebsiella isolate carries a gene that encodes an enzyme adding an acetyl group to gentamicin. How does this make the isolate resistant?

AThe drug is enzymatically modified and can no longer bind the ribosome effectivelyBThe drug is pumped out of the cell before it actsCThe drug target on the 30S subunit is methylated or mutatedDThe drug fails to enter because porin channels are lost

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

The answer and the reasoning

A. The drug is enzymatically modified and can no longer bind the ribosome effectively

Aminoglycoside-modifying enzymes such as acetyltransferases, phosphotransferases and adenylyltransferases chemically alter the antibiotic so that it no longer binds the ribosomal target. These genes are commonly plasmid-borne and transferable. It is the most common mechanism of aminoglycoside resistance.

Option by option

A. The drug is enzymatically modified and can no longer bind the ribosome effectively (correct)
Correct. Acetylation inactivates the drug so it cannot bind its ribosomal target.
B. The drug is pumped out of the cell before it acts
Efflux is a distinct mechanism that exports the drug unchanged.
C. The drug target on the 30S subunit is methylated or mutated
Ribosomal modification is a target-alteration mechanism and is not caused by an acetyltransferase.
D. The drug fails to enter because porin channels are lost
Porin loss reduces entry and is separate from an enzyme acting on the 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 A, The drug is enzymatically modified and can no longer bind the ribosome effectively. Aminoglycoside-modifying enzymes such as acetyltransferases, phosphotransferases and adenylyltransferases chemically alter the antibiotic so that it no longer binds the ribosomal target.

Why is "The drug is pumped out of the cell before it acts" not the answer?

Efflux is a distinct mechanism that exports the drug unchanged.

Why is "The drug target on the 30S subunit is methylated or mutated" not the answer?

Ribosomal modification is a target-alteration mechanism and is not caused by an acetyltransferase.

Why is "The drug fails to enter because porin channels are lost" not the answer?

Porin loss reduces entry and is separate from an enzyme acting on the drug.

What concept does this question test?

It tests General microbiology and sterilisation: aminoglycoside-modifying enzymes in Microbiology. These genes are commonly plasmid-borne and transferable. It is the most common mechanism of aminoglycoside resistance.

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 General microbiology and sterilisation: aminoglycoside-modifying enzymes?

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.