Bacteriology: nalidixic acid resistance in typhoid: 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 · Bacteriology: nalidixic acid resistance in typhoid

Sample item

A patient with culture-confirmed typhoid shows no fever response after 5 days of ciprofloxacin. The isolate is resistant to nalidixic acid with ciprofloxacin MIC in the intermediate range. What does this indicate?

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

The answer and the reasoning

B. Reduced fluoroquinolone susceptibility from a gyrA mutation

Nalidixic acid resistance is a marker of a gyrA mutation, causing reduced susceptibility to ciprofloxacin and treatment failure. Azithromycin or ceftriaxone is then used. Beta-lactamase and ribosomal methylation do not account for quinolone resistance.

Option by option

  • A. Beta-lactamase production

    Beta-lactamase affects penicillins and cephalosporins, not quinolones.

  • B. Reduced fluoroquinolone susceptibility from a gyrA mutation (correct)

    Correct. Nalidixic acid resistance flags a gyrA mutation.

  • C. Loss of the Vi capsule

    Vi capsule loss does not affect quinolone susceptibility.

  • D. Ribosomal methylation

    Ribosomal methylation causes macrolide and aminoglycoside resistance.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Reduced fluoroquinolone susceptibility from a gyrA mutation. Nalidixic acid resistance is a marker of a gyrA mutation, causing reduced susceptibility to ciprofloxacin and treatment failure.

Why is "Beta-lactamase production" not the answer?

Beta-lactamase affects penicillins and cephalosporins, not quinolones.

Why is "Loss of the Vi capsule" not the answer?

Vi capsule loss does not affect quinolone susceptibility.

Why is "Ribosomal methylation" not the answer?

Ribosomal methylation causes macrolide and aminoglycoside resistance.

What concept does this question test?

It tests Bacteriology: nalidixic acid resistance in typhoid in Microbiology. Azithromycin or ceftriaxone is then used. Beta-lactamase and ribosomal methylation do not account for quinolone resistance.

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: nalidixic acid resistance in typhoid?

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.