Bacteriology: biological false-positive VDRL: 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 · Bacteriology: biological false-positive VDRL

Sample item

A 28-year-old woman with systemic lupus erythematosus and recurrent miscarriages has a reactive VDRL at a titre of 1 in 8. TPHA and FTA-ABS are both non-reactive and she has no history of genital ulcer. What explains the VDRL result?

AAntiphospholipid antibodies reacting with the cardiolipin antigenBCross-reaction with Leptospira antigenCProzone phenomenon from antibody excessDPrimary syphilis in the window period

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

The answer and the reasoning

A. Antiphospholipid antibodies reacting with the cardiolipin antigen

VDRL uses a cardiolipin-lecithin-cholesterol antigen. Antiphospholipid antibodies of lupus, as well as pregnancy, malaria, leprosy and other conditions, can give a biological false-positive reaction, which a negative specific treponemal test exposes.

Option by option

A. Antiphospholipid antibodies reacting with the cardiolipin antigen (correct)
Correct. Antiphospholipid antibodies bind the cardiolipin reagent and give a biological false positive.
B. Cross-reaction with Leptospira antigen
Nothing in the history suggests leptospirosis, and lupus with miscarriages explains the result.
C. Prozone phenomenon from antibody excess
A prozone effect gives a false-negative result in neat serum, not a reactive one.
D. Primary syphilis in the window period
Early primary syphilis would not explain nonreactive treponemal tests in a woman without ulcer history and with lupus.

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 A, Antiphospholipid antibodies reacting with the cardiolipin antigen. VDRL uses a cardiolipin-lecithin-cholesterol antigen.

Why is "Cross-reaction with Leptospira antigen" not the answer?

Nothing in the history suggests leptospirosis, and lupus with miscarriages explains the result.

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

A prozone effect gives a false-negative result in neat serum, not a reactive one.

Why is "Primary syphilis in the window period" not the answer?

Early primary syphilis would not explain nonreactive treponemal tests in a woman without ulcer history and with lupus.

What concept does this question test?

It tests Bacteriology: biological false-positive VDRL in Microbiology. Antiphospholipid antibodies of lupus, as well as pregnancy, malaria, leprosy and other conditions, can give a biological false-positive reaction, which a negative specific treponemal test exposes.

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 Bacteriology: biological false-positive VDRL?

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.