Microscopy: fluorescent acid-fast staining: 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 · Microscopy: fluorescent acid-fast staining

Sample item

A busy laboratory under the national TB programme chooses auramine-rhodamine fluorescence microscopy to screen sputum smears. What is its main advantage over Ziehl-Neelsen microscopy?

AIt stains only viable bacilliBIt distinguishes Mycobacterium tuberculosis from non-tuberculous mycobacteriaCSmears can be scanned at lower magnification, which is faster and increases sensitivityDIt requires no decolourisation step

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

The answer and the reasoning

C. Smears can be scanned at lower magnification, which is faster and increases sensitivity

Fluorescing bacilli are visible against a dark background at 20x to 40x, so a much larger area can be scanned in less time. This improves throughput and sensitivity compared with oil immersion examination. The method still needs acid-alcohol decolourisation, cannot identify species and stains dead and live bacilli alike.

Option by option

A. It stains only viable bacilli
The stain binds mycolic acids in dead and live bacilli alike and cannot assess viability.
B. It distinguishes Mycobacterium tuberculosis from non-tuberculous mycobacteria
Auramine-rhodamine cannot separate M. tuberculosis from non-tuberculous mycobacteria.
C. Smears can be scanned at lower magnification, which is faster and increases sensitivity (correct)
Correct. Scanning at lower magnification is quicker and detects more positive smears.
D. It requires no decolourisation step
A decolourisation step with acid-alcohol is still required, followed by a counterstain.

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, Smears can be scanned at lower magnification, which is faster and increases sensitivity. Fluorescing bacilli are visible against a dark background at 20x to 40x, so a much larger area can be scanned in less time.

Why is "It stains only viable bacilli" not the answer?

The stain binds mycolic acids in dead and live bacilli alike and cannot assess viability.

Why is "It distinguishes Mycobacterium tuberculosis from non-tuberculous myco…" not the answer?

Auramine-rhodamine cannot separate M. tuberculosis from non-tuberculous mycobacteria.

Why is "It requires no decolourisation step" not the answer?

A decolourisation step with acid-alcohol is still required, followed by a counterstain.

What concept does this question test?

It tests Microscopy: fluorescent acid-fast staining in Microbiology. This improves throughput and sensitivity compared with oil immersion examination. The method still needs acid-alcohol decolourisation, cannot identify species and stains dead and live bacilli alike.

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 Microscopy: fluorescent acid-fast staining?

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.