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Syphilis — SCE Dermatology MCQ

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HardCutaneous InfectionSyphilisSCE Dermatology

A man has a diffuse papulosquamous eruption, mucous patches and generalized lymphadenopathy. Treponemal serology is positive but the laboratory reports an undiluted RPR as non-reactive. Which request best resolves the discordance?

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Correct answer: E — Repeat RPR on serial serum dilutions to reveal a prozone reaction

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

C is correct. In florid secondary syphilis, very high antibody concentrations can interfere with lattice formation in an undiluted nontreponemal assay, producing a false-negative prozone result. Serial dilution restores visible reactivity and provides a baseline titre for monitoring. A reactive treponemal test usually remains reactive long term and repeating it does not resolve whether current disease is active. Histology may support but cannot replace serology and clinical assessment. The presentation warrants prompt specialist sexual-health management, HIV testing, partner notification and treatment rather than waiting for an arbitrary seroconversion interval.

Reference: BASHH UK guideline for syphilis: https://www.bashh.org/_userfiles/pages/files/resources/uk_syphilis_guidelines_2015.pdf