General microbiology and sterilisation: tellurite medium: 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 · General microbiology and sterilisation: tellurite medium

Sample item

A throat swab from a child with a grey pharyngeal membrane is plated on blood agar containing potassium tellurite. After 48 hours, grey-black colonies of club-shaped Gram-positive rods are seen. Why are the colonies dark?

AThe organism produces hydrogen sulphide that precipitates iron saltsBThe organism ferments lactose and takes up neutral redCThe organism lyses red cells and forms methaemoglobinDThe organism reduces tellurite to metallic tellurium within its cells

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

The answer and the reasoning

D. The organism reduces tellurite to metallic tellurium within its cells

Corynebacterium diphtheriae reduces potassium tellurite to metallic tellurium, which deposits in the colony as grey to black. Tellurite also inhibits most other throat flora, making the medium selective. This is the principle behind tellurite blood agar for diphtheria.

Option by option

A. The organism produces hydrogen sulphide that precipitates iron salts
Hydrogen sulphide production with iron salts is the principle of TSI and similar media.
B. The organism ferments lactose and takes up neutral red
Lactose fermentation with neutral red is the basis of MacConkey agar colour, not of tellurite medium.
C. The organism lyses red cells and forms methaemoglobin
Haemolysis produces clearing or greenish zones on blood agar and does not account for black colonies.
D. The organism reduces tellurite to metallic tellurium within its cells (correct)
Correct. Intracellular reduction of tellurite to metallic tellurium blackens the colonies.

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 D, The organism reduces tellurite to metallic tellurium within its cells. Corynebacterium diphtheriae reduces potassium tellurite to metallic tellurium, which deposits in the colony as grey to black.

Why is "The organism produces hydrogen sulphide that precipitates iron salts" not the answer?

Hydrogen sulphide production with iron salts is the principle of TSI and similar media.

Why is "The organism ferments lactose and takes up neutral red" not the answer?

Lactose fermentation with neutral red is the basis of MacConkey agar colour, not of tellurite medium.

Why is "The organism lyses red cells and forms methaemoglobin" not the answer?

Haemolysis produces clearing or greenish zones on blood agar and does not account for black colonies.

What concept does this question test?

It tests General microbiology and sterilisation: tellurite medium in Microbiology. Tellurite also inhibits most other throat flora, making the medium selective. This is the principle behind tellurite blood agar for diphtheria.

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 and sterilisation: tellurite medium?

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.