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Syphilis HIV — SCE Dermatology MCQ

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HardCutaneous InfectionSyphilis HIVSCE Dermatology

A man with HIV has fever, generalised lymphadenopathy, a palmoplantar papulosquamous eruption and condylomata lata. An undiluted rapid plasma reagin test is reported negative. What laboratory step best addresses the discordance?

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Correct answer: C — Repeat the non-treponemal assay on serially diluted serum and perform treponemal testing

Explanation lettering: D = shown as A · A = shown as B · B = shown as C · E = shown as D · C = shown as E

B is correct. The phenotype is strongly suggestive of secondary syphilis. At very high antibody concentrations, an undiluted non-treponemal assay may fail to form the visible antigen-antibody lattice: the prozone phenomenon. Serial dilution moves the specimen into the assay’s reactive range, while a treponemal assay provides a complementary test. Repeating the identical undiluted method preserves the analytical problem; total IgM does not resolve it; a routine bacterial swab cannot exclude Treponema pallidum; and HIV does not make secondary syphilis impossible. The laboratory should be told the clinical suspicion so that appropriate dilution and confirmatory testing are undertaken.

Reference: BASHH UK national guideline for the management of syphilis 2024: https://www.bashh.org/_userfiles/pages/files/syphilis_2024.pdf