A patient with a widespread maculopapular rash suggestive of secondary syphilis has a non-reactive undiluted VDRL, but the test becomes reactive when the serum is diluted. What explains this finding?
- A Prozone phenomenon from excess antibody blocking flocculation ✓
- B Biological false positive due to antiphospholipid antibodies
- C Seroreversion after prior treated infection
- D False negative because nontreponemal tests lack sensitivity in secondary syphilis
Correct answer: A. Prozone phenomenon from excess antibody blocking flocculation
Explanation
The prozone phenomenon occurs when very high antibody titres interfere with antigen-antibody flocculation, producing a false non-reactive result in undiluted serum; dilution resolves it. It is classically seen in secondary syphilis and pregnancy. Nontreponemal tests are actually highly sensitive in secondary syphilis, which makes option D untenable.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.