Clinical microbiology and hospital infection control: paired sera: 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 · Clinical microbiology and hospital infection control: paired sera

Sample item

A patient has a febrile illness and a single serum drawn at presentation shows a moderately raised IgG titre to the suspected pathogen. What is the best way to confirm acute infection by serology?

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

The answer and the reasoning

A. Convalescent serum after 2 weeks showing a four-fold titre rise

A single raised IgG may reflect past exposure. Paired sera, acute and convalescent about 2 weeks apart and tested together, with at least a four-fold rise in titre, confirm recent infection.

Option by option

  • A. Convalescent serum after 2 weeks showing a four-fold titre rise (correct)

    Correct. A four-fold rise between paired sera indicates acute infection.

  • B. The same sample tested again with another kit

    Retesting the same sample cannot show a change over time.

  • C. A higher dilution of the first sample only

    Further dilution of the first sample gives no information on a rising titre.

  • D. A repeat sample after 24 hours showing any rise

    Antibody titres do not change meaningfully within 24 hours.

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, Convalescent serum after 2 weeks showing a four-fold titre rise. A single raised IgG may reflect past exposure.

Why is "The same sample tested again with another kit" not the answer?

Retesting the same sample cannot show a change over time.

Why is "A higher dilution of the first sample only" not the answer?

Further dilution of the first sample gives no information on a rising titre.

Why is "A repeat sample after 24 hours showing any rise" not the answer?

Antibody titres do not change meaningfully within 24 hours.

What concept does this question test?

It tests Clinical microbiology and hospital infection control: paired sera in Microbiology. Paired sera, acute and convalescent about 2 weeks apart and tested together, with at least a four-fold rise in titre, confirm recent infection.

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 Clinical microbiology and hospital infection control: paired sera?

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.