Virology: haemagglutination inhibition test: 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 · Virology: haemagglutination inhibition test

Sample item

To measure antibodies to influenza virus, a serum sample is incubated with a standard amount of virus, and red blood cells are then added. No haemagglutination occurs in the well. What does this indicate?

ASerum neuraminidase degraded the virus particlesBThe virus was inactivated by serum complementCThe serum contains no antibody to the virusDSerum antibodies blocked the viral haemagglutinin

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

The answer and the reasoning

D. Serum antibodies blocked the viral haemagglutinin

In the haemagglutination inhibition test, antibodies that bind the haemagglutinin prevent the virus from cross-linking red cells, so the cells settle as a button. The highest serum dilution that inhibits agglutination is the titre, and a fourfold rise in paired sera suggests recent infection.

Option by option

A. Serum neuraminidase degraded the virus particles
The serum does not contain neuraminidase that destroys virions, and this is not the principle of the test.
B. The virus was inactivated by serum complement
Complement is not added or required in this test.
C. The serum contains no antibody to the virus
If there were no antibody, the virus would agglutinate the red cells.
D. Serum antibodies blocked the viral haemagglutinin (correct)
Correct. Antibody bound to haemagglutinin prevents the virus from agglutinating red cells.

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, Serum antibodies blocked the viral haemagglutinin. In the haemagglutination inhibition test, antibodies that bind the haemagglutinin prevent the virus from cross-linking red cells, so the cells settle as a button.

Why is "Serum neuraminidase degraded the virus particles" not the answer?

The serum does not contain neuraminidase that destroys virions, and this is not the principle of the test.

Why is "The virus was inactivated by serum complement" not the answer?

Complement is not added or required in this test.

Why is "The serum contains no antibody to the virus" not the answer?

If there were no antibody, the virus would agglutinate the red cells.

What concept does this question test?

It tests Virology: haemagglutination inhibition test in Microbiology. The highest serum dilution that inhibits agglutination is the titre, and a fourfold rise in paired sera suggests 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 Virology: haemagglutination inhibition test?

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.