STI: prozone phenomenon: practice question

A Dermatology 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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Dermatology · STI: prozone phenomenon

Sample item

A patient has a generalised rash with palmar lesions typical of secondary syphilis, yet the undiluted VDRL is nonreactive. What is the explanation?

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

The answer and the reasoning

D. Prozone phenomenon due to antibody excess, which is corrected by serum dilution

In secondary syphilis, very high antibody levels may prevent flocculation by excess antibody (prozone), producing a false negative result. Repeating the test with diluted serum gives a reactive result.

Option by option

  • A. Technical error in the antigen only

    Technical error is possible, but the prozone effect is the classic explanation.

  • B. Immunity to treponemes

    Immunity to treponemes does not give a negative test during active disease.

  • C. Latent syphilis

    Latent syphilis lacks a rash.

  • D. Prozone phenomenon due to antibody excess, which is corrected by serum dilution (correct)

    Correct. Antibody excess causes the prozone effect.

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 Dermatology question

What is the answer to this Dermatology question?

The best answer is D, Prozone phenomenon due to antibody excess, which is corrected by serum dilution. In secondary syphilis, very high antibody levels may prevent flocculation by excess antibody (prozone), producing a false negative result.

Why is "Technical error in the antigen only" not the answer?

Technical error is possible, but the prozone effect is the classic explanation.

Why is "Immunity to treponemes" not the answer?

Immunity to treponemes does not give a negative test during active disease.

Why is "Latent syphilis" not the answer?

Latent syphilis lacks a rash.

What concept does this question test?

It tests STI: prozone phenomenon in Dermatology. Repeating the test with diluted serum gives a reactive result.

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 STI: prozone phenomenon?

Learn each condition by its primary lesion, site and signature sign. Then use the flashcards and the other Dermatology 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.