Bacteriology: katG isoniazid resistance: 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: katG isoniazid resistance

Sample item

A Mycobacterium tuberculosis isolate shows high-level isoniazid resistance. Sequencing finds a mutation in the katG gene. How does this mutation cause resistance?

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

The answer and the reasoning

A. Loss of the catalase-peroxidase that activates the prodrug

Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase encoded by katG. Mutations such as S315T reduce activation and give high-level resistance, whereas inhA promoter mutations cause lower-level resistance.

Option by option

  • A. Loss of the catalase-peroxidase that activates the prodrug (correct)

    Correct. Without the activating enzyme, isoniazid never becomes active.

  • B. Loss of pyrazinamidase activity

    Loss of pyrazinamidase (pncA mutation) causes pyrazinamide resistance.

  • C. Altered arabinosyl transferase enzyme

    Altered arabinosyl transferase (embB) causes ethambutol resistance.

  • D. A mutated RNA polymerase beta subunit

    A mutated RNA polymerase beta subunit (rpoB) causes rifampicin 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 A, Loss of the catalase-peroxidase that activates the prodrug. Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase encoded by katG.

Why is "Loss of pyrazinamidase activity" not the answer?

Loss of pyrazinamidase (pncA mutation) causes pyrazinamide resistance.

Why is "Altered arabinosyl transferase enzyme" not the answer?

Altered arabinosyl transferase (embB) causes ethambutol resistance.

Why is "A mutated RNA polymerase beta subunit" not the answer?

A mutated RNA polymerase beta subunit (rpoB) causes rifampicin resistance.

What concept does this question test?

It tests Bacteriology: katG isoniazid resistance in Microbiology. Mutations such as S315T reduce activation and give high-level resistance, whereas inhA promoter mutations cause lower-level 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 Bacteriology: katG isoniazid resistance?

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.