Bacteriology: vancomycin-resistant enterococci: 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.

Try it

Microbiology · Bacteriology: vancomycin-resistant enterococci

Sample item

An ICU patient has bacteraemia with Enterococcus faecium that is resistant to ampicillin and to vancomycin at high level, with the vanA gene detected. By which mechanism does vanA produce resistance?

AIt methylates the 23S ribosomal RNA binding siteBIt pumps vancomycin out through a membrane efflux systemCIt makes peptidoglycan precursors end in D-Ala-D-Lac instead of D-Ala-D-AlaDIt produces a beta-lactamase that hydrolyses vancomycin

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

The answer and the reasoning

C. It makes peptidoglycan precursors end in D-Ala-D-Lac instead of D-Ala-D-Ala

Vancomycin binds the D-Ala-D-Ala terminus of the pentapeptide to block cell wall cross-linking. The vanA operon encodes enzymes that make D-Ala-D-Lac precursors, which bind vancomycin with about 1000-fold lower affinity, producing high-level resistance, often to teicoplanin as well. Linezolid or daptomycin is used for treatment.

Option by option

A. It methylates the 23S ribosomal RNA binding site
Ribosomal RNA methylation by erm genes causes macrolide and clindamycin resistance, not vancomycin resistance.
B. It pumps vancomycin out through a membrane efflux system
Efflux is a mechanism of tetracycline resistance and is not how vanA acts.
C. It makes peptidoglycan precursors end in D-Ala-D-Lac instead of D-Ala-D-Ala (correct)
Correct. Target modification of the peptidoglycan precursor explains the resistance.
D. It produces a beta-lactamase that hydrolyses vancomycin
Vancomycin is not hydrolysed by beta-lactamase, which acts on penicillins and cephalosporins.

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, It makes peptidoglycan precursors end in D-Ala-D-Lac instead of D-Ala-D-Ala. Vancomycin binds the D-Ala-D-Ala terminus of the pentapeptide to block cell wall cross-linking.

Why is "It methylates the 23S ribosomal RNA binding site" not the answer?

Ribosomal RNA methylation by erm genes causes macrolide and clindamycin resistance, not vancomycin resistance.

Why is "It pumps vancomycin out through a membrane efflux system" not the answer?

Efflux is a mechanism of tetracycline resistance and is not how vanA acts.

Why is "It produces a beta-lactamase that hydrolyses vancomycin" not the answer?

Vancomycin is not hydrolysed by beta-lactamase, which acts on penicillins and cephalosporins.

What concept does this question test?

It tests Bacteriology: vancomycin-resistant enterococci in Microbiology. The vanA operon encodes enzymes that make D-Ala-D-Lac precursors, which bind vancomycin with about 1000-fold lower affinity, producing high-level resistance, often to teicoplanin as well. Linezolid or daptomycin is used for treatment.

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 Bacteriology: vancomycin-resistant enterococci?

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.