Salmonella Typhi: Vi antigen: 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 · Salmonella Typhi: Vi antigen

Sample item

A Salmonella Typhi isolate from blood fails to agglutinate with group D (O9) antiserum on a slide. After the bacterial suspension is boiled and cooled, it agglutinates strongly. What explains this?

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

The answer and the reasoning

D. The surface Vi capsular antigen masked the O antigen

The Vi antigen is a capsular polysaccharide of Salmonella Typhi that can block access of O antiserum to the somatic antigen. Heating removes or inactivates Vi, exposing the heat-stable O antigen, so agglutination then occurs. H antigens are heat-labile.

Option by option

  • A. Mutation making the organism a different genus

    A different genus would not react after boiling.

  • B. Prozone effect from excess antibody

    Prozone arises from excess antibody and would not be corrected by boiling.

  • C. Loss of the H flagellar antigen by phase variation

    H antigen is flagellar and heat-labile, and its loss would not affect O agglutination.

  • D. The surface Vi capsular antigen masked the O antigen (correct)

    Correct. Boiling removes the masking Vi antigen and exposes O antigen.

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 D, The surface Vi capsular antigen masked the O antigen. The Vi antigen is a capsular polysaccharide of Salmonella Typhi that can block access of O antiserum to the somatic antigen.

Why is "Mutation making the organism a different genus" not the answer?

A different genus would not react after boiling.

Why is "Prozone effect from excess antibody" not the answer?

Prozone arises from excess antibody and would not be corrected by boiling.

Why is "Loss of the H flagellar antigen by phase variation" not the answer?

H antigen is flagellar and heat-labile, and its loss would not affect O agglutination.

What concept does this question test?

It tests Salmonella Typhi: Vi antigen in Microbiology. Heating removes or inactivates Vi, exposing the heat-stable O antigen, so agglutination then occurs. H antigens are heat-labile.

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 Salmonella Typhi: Vi antigen?

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.