Clinical microbiology: CBNAAT result: 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 · Clinical microbiology: CBNAAT result

Sample item

A 38-year-old man with cough for six weeks has sputum tested by CBNAAT (Xpert MTB/RIF), which reports MTB detected, rifampicin resistance detected. What is the most appropriate next step under the national programme?

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

The answer and the reasoning

A. Test further for isoniazid and fluoroquinolone resistance (for example by line probe assay) and refer to a DR-TB centre

CBNAAT detects Mycobacterium tuberculosis and mutations in the rpoB gene that confer rifampicin resistance. Rifampicin resistance is treated as probable multidrug-resistant TB, so the next step is to assess resistance to isoniazid and fluoroquinolones, using line probe assay or culture-based drug susceptibility testing, and to manage at a drug-resistant TB centre with a regimen based on the result.

Option by option

  • A. Test further for isoniazid and fluoroquinolone resistance (for example by line probe assay) and refer to a DR-TB centre (correct)

    Further testing for isoniazid and fluoroquinolone resistance guides the drug-resistant regimen.

  • B. Repeat CBNAAT every week until it becomes negative

    Repeating the same test will not change the resistance result and delays correct treatment.

  • C. Start standard four-drug first-line therapy and review in two months

    Standard first-line therapy relies on rifampicin, so it would be inadequate and would amplify resistance.

  • D. Add streptomycin to first-line therapy and continue

    Adding a single drug to a failing regimen is unsound and does not treat rifampicin-resistant TB.

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, Test further for isoniazid and fluoroquinolone resistance (for example by line probe assay) and refer to a DR-TB centre. CBNAAT detects Mycobacterium tuberculosis and mutations in the rpoB gene that confer rifampicin resistance.

Why is "Repeat CBNAAT every week until it becomes negative" not the answer?

Repeating the same test will not change the resistance result and delays correct treatment.

Why is "Start standard four-drug first-line therapy and review in two months" not the answer?

Standard first-line therapy relies on rifampicin, so it would be inadequate and would amplify resistance.

Why is "Add streptomycin to first-line therapy and continue" not the answer?

Adding a single drug to a failing regimen is unsound and does not treat rifampicin-resistant TB.

What concept does this question test?

It tests Clinical microbiology: CBNAAT result in Microbiology. Rifampicin resistance is treated as probable multidrug-resistant TB, so the next step is to assess resistance to isoniazid and fluoroquinolones, using line probe assay or culture-based drug susceptibility testing, and to manage at a drug-resistant TB centre with a regimen based on the result.

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 Clinical microbiology: CBNAAT result?

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.