Staining: acid-fast staining of Mycobacterium leprae: 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 · Staining: acid-fast staining of Mycobacterium leprae

Sample item

A slit-skin smear from a lepromatous nodule is treated with 20% sulfuric acid in a standard Ziehl-Neelsen stain and reported negative, yet the patient clearly has leprosy. What is the likely technical reason?

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

The answer and the reasoning

C. Mycobacterium leprae is weakly acid-fast and is decolourised by 20% acid, so 5% sulfuric acid is used

M. leprae has a less robust, lipid-rich wall than M. tuberculosis and loses carbol fuchsin with strong acid. For leprosy smears the decolourising agent is 5% sulfuric acid. Nocardia and some parasitic oocysts need even weaker acid (modified acid-fast staining, 1%).

Option by option

  • A. Mycobacterium leprae stains only with Gram stain and not with carbol fuchsin

    M. leprae takes up carbol fuchsin, and Gram staining is unreliable for mycobacteria.

  • B. Mycobacterium leprae is not acid-fast because it lacks mycolic acids entirely

    M. leprae does contain mycolic acids and is acid-fast, though weakly.

  • C. Mycobacterium leprae is weakly acid-fast and is decolourised by 20% acid, so 5% sulfuric acid is used (correct)

    Correct. Weak acid-fastness means 5% sulfuric acid must be used.

  • D. Slit-skin smears require 1% sulfuric acid, which is the strength used for Cryptosporidium

    One percent acid is for modified acid-fast organisms such as Nocardia, not the usual leprosy method.

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 C, Mycobacterium leprae is weakly acid-fast and is decolourised by 20% acid, so 5% sulfuric acid is used. M.

Why is "Mycobacterium leprae stains only with Gram stain and not with carbol…" not the answer?

M. leprae takes up carbol fuchsin, and Gram staining is unreliable for mycobacteria.

Why is "Mycobacterium leprae is not acid-fast because it lacks mycolic acids…" not the answer?

M. leprae does contain mycolic acids and is acid-fast, though weakly.

Why is "Slit-skin smears require 1% sulfuric acid, which is the strength used…" not the answer?

One percent acid is for modified acid-fast organisms such as Nocardia, not the usual leprosy method.

What concept does this question test?

It tests Staining: acid-fast staining of Mycobacterium leprae in Microbiology. leprae has a less robust, lipid-rich wall than M. tuberculosis and loses carbol fuchsin with strong acid. For leprosy smears the decolourising agent is 5% sulfuric acid. Nocardia and some parasitic oocysts need even weaker acid (modified acid-fast staining, 1%).

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 Staining: acid-fast staining of Mycobacterium leprae?

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.