General microbiology: special stains: 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.

Try it

Microbiology · General microbiology: special stains

Sample item

A man has a painless genital ulcer and the laboratory wants to visualise the causative spirochaete in a smear from the lesion. The organism is too thin to be seen with Gram stain. Which preparation deposits metallic silver on the organisms to make them visible?

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

The answer and the reasoning

B. Fontana stain

Treponema pallidum is about 0.15 micrometre wide and falls below the resolution of ordinary stains. Silver impregnation methods such as Fontana or Levaditi coat the spirochaete and thicken it, and dark-field microscopy is the alternative for live organisms.

Option by option

  • A. Ziehl-Neelsen stain

    Ziehl-Neelsen stain demonstrates acid-fast bacilli such as mycobacteria.

  • B. Fontana stain (correct)

    Correct. Fontana is a silver impregnation stain for spirochaetes.

  • C. India ink preparation

    India ink creates a negative stain that outlines the capsule of Cryptococcus.

  • D. Albert stain

    Albert stain demonstrates the metachromatic granules of Corynebacterium diphtheriae.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Fontana stain. Treponema pallidum is about 0.

Why is "Ziehl-Neelsen stain" not the answer?

Ziehl-Neelsen stain demonstrates acid-fast bacilli such as mycobacteria.

Why is "India ink preparation" not the answer?

India ink creates a negative stain that outlines the capsule of Cryptococcus.

Why is "Albert stain" not the answer?

Albert stain demonstrates the metachromatic granules of Corynebacterium diphtheriae.

What concept does this question test?

It tests General microbiology: special stains in Microbiology. 15 micrometre wide and falls below the resolution of ordinary stains. Silver impregnation methods such as Fontana or Levaditi coat the spirochaete and thicken it, and dark-field microscopy is the alternative for live organisms.

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 General microbiology: special stains?

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.