Immunology: serological techniques: 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 · Immunology: serological techniques

Sample item

A man has a painless genital ulcer healing, a diffuse rash and generalised lymphadenopathy, and a neat-serum VDRL is non-reactive. When the serum is diluted 1 in 8, VDRL becomes reactive. What explains this?

APostzone phenomenon from excess antigenBBiological false-positive reaction from lupusCProzone phenomenon from excess antibody preventing lattice formationDWindow period before antibodies appear

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

The answer and the reasoning

C. Prozone phenomenon from excess antibody preventing lattice formation

In the prozone phenomenon, very high antibody concentration saturates antigen epitopes so that cross-linked lattices cannot form, giving a false-negative flocculation reaction. It is seen in secondary syphilis, and diluting the serum restores reactivity. Suspected cases should be tested at dilution.

Option by option

A. Postzone phenomenon from excess antigen
Postzone is due to antigen excess, and dilution would not convert a negative to positive.
B. Biological false-positive reaction from lupus
A false positive from lupus would not be negative on neat serum.
C. Prozone phenomenon from excess antibody preventing lattice formation (correct)
Correct. Dilution restores the antigen-antibody balance and the test turns reactive.
D. Window period before antibodies appear
A window period would stay negative on dilution, since no antibody is present.

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 C, Prozone phenomenon from excess antibody preventing lattice formation. In the prozone phenomenon, very high antibody concentration saturates antigen epitopes so that cross-linked lattices cannot form, giving a false-negative flocculation reaction.

Why is "Postzone phenomenon from excess antigen" not the answer?

Postzone is due to antigen excess, and dilution would not convert a negative to positive.

Why is "Biological false-positive reaction from lupus" not the answer?

A false positive from lupus would not be negative on neat serum.

Why is "Window period before antibodies appear" not the answer?

A window period would stay negative on dilution, since no antibody is present.

What concept does this question test?

It tests Immunology: serological techniques in Microbiology. It is seen in secondary syphilis, and diluting the serum restores reactivity. Suspected cases should be tested at dilution.

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 Immunology: serological techniques?

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.