Bacteriology: scarlet fever exotoxin: 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 · Bacteriology: scarlet fever exotoxin

Sample item

A 7-year-old has sore throat, fever, a fine sandpaper rash sparing the area around the mouth, and a strawberry tongue. A throat swab grows beta-haemolytic, bacitracin-sensitive Gram-positive cocci in chains. Which factor produces the rash?

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

The answer and the reasoning

A. Phage-encoded pyrogenic exotoxin (superantigen)

Scarlet fever is caused by group A streptococci lysogenised by a bacteriophage carrying the gene for a streptococcal pyrogenic exotoxin such as SpeA or SpeC. The exotoxin acts as a superantigen, and the rash appears only in people without antitoxin immunity.

Option by option

  • A. Phage-encoded pyrogenic exotoxin (superantigen) (correct)

    Correct. The erythrogenic toxin is phage-coded and causes the rash by superantigen action.

  • B. Streptolysin O, lysing leucocytes

    Streptolysin O lyses cells and is the basis of the ASO test, but it does not cause the rash.

  • C. M protein, blocking complement deposition

    M protein resists phagocytosis and is the basis of type specificity.

  • D. Hyaluronidase, degrading connective tissue

    Hyaluronidase helps spread through tissues and does not cause the rash.

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, Phage-encoded pyrogenic exotoxin (superantigen). Scarlet fever is caused by group A streptococci lysogenised by a bacteriophage carrying the gene for a streptococcal pyrogenic exotoxin such as SpeA or SpeC.

Why is "Streptolysin O, lysing leucocytes" not the answer?

Streptolysin O lyses cells and is the basis of the ASO test, but it does not cause the rash.

Why is "M protein, blocking complement deposition" not the answer?

M protein resists phagocytosis and is the basis of type specificity.

Why is "Hyaluronidase, degrading connective tissue" not the answer?

Hyaluronidase helps spread through tissues and does not cause the rash.

What concept does this question test?

It tests Bacteriology: scarlet fever exotoxin in Microbiology. The exotoxin acts as a superantigen, and the rash appears only in people without antitoxin immunity.

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 Bacteriology: scarlet fever exotoxin?

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.