Streptococcus pyogenes: serology after skin infection: 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 · Streptococcus pyogenes: serology after skin infection

Sample item

A 7-year-old develops cola-coloured urine, oedema and hypertension three weeks after impetigo. Antistreptolysin O titre is normal. Which test best supports a preceding streptococcal infection?

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

The answer and the reasoning

C. Anti-DNase B titre

After streptococcal skin infection, the ASO response is often weak because skin lipids inactivate streptolysin O. Anti-DNase B and anti-hyaluronidase antibodies rise reliably after both throat and skin infection and can show a preceding infection in post-streptococcal glomerulonephritis. Low complement C3 supports the diagnosis.

Option by option

  • A. Repeat ASO titre only

    ASO is characteristically low after skin infection, so repeating it is unlikely to help.

  • B. Widal test

    The Widal test is for enteric fever.

  • C. Anti-DNase B titre (correct)

    Correct. Anti-DNase B is raised after skin infection when ASO is not.

  • D. Paul-Bunnell test

    The Paul-Bunnell test detects heterophile antibodies in infectious mononucleosis.

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, Anti-DNase B titre. After streptococcal skin infection, the ASO response is often weak because skin lipids inactivate streptolysin O.

Why is "Repeat ASO titre only" not the answer?

ASO is characteristically low after skin infection, so repeating it is unlikely to help.

Why is "Widal test" not the answer?

The Widal test is for enteric fever.

Why is "Paul-Bunnell test" not the answer?

The Paul-Bunnell test detects heterophile antibodies in infectious mononucleosis.

What concept does this question test?

It tests Streptococcus pyogenes: serology after skin infection in Microbiology. Anti-DNase B and anti-hyaluronidase antibodies rise reliably after both throat and skin infection and can show a preceding infection in post-streptococcal glomerulonephritis. Low complement C3 supports the diagnosis.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Streptococcus pyogenes: serology after skin infection?

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.