A 30-year-old sex worker presents with a painless ulcer on the penis (chancre) for 10 days. VDRL test is non-reactive. FTA-ABS (fluorescent treponemal antibody absorption) test is reactive. The interpretation of this discordance is MOST likely:

• A Biological false-positive VDRL due to yaws cross-reaction, with truly positive FTA-ABS confirming syphilis
• B Prozone phenomenon — excess VDRL antibodies preventing visible agglutination; dilution will show positivity
• C Late latent syphilis — VDRL titers spontaneously fall below detection while FTA-ABS remains positive for life
• D Early primary syphilis — VDRL (non-treponemal) becomes positive 1–4 weeks after chancre; FTA-ABS (treponemal) becomes positive earlier and may be the only positive test in early primary syphilis ✓

Explanation

In primary syphilis, the temporal relationship of serological tests is critical: FTA-ABS (treponemal test) becomes reactive first — typically within 3–4 weeks of infection and before or around the time of chancre appearance. VDRL/RPR (non-treponemal tests) becomes reactive 1–4 weeks after the chancre appears (4–8 weeks post-infection). In early primary syphilis (<2–3 weeks from chancre), FTA-ABS can be the only positive serological test. This is why a reactive FTA-ABS with non-reactive VDRL during active ulcer is diagnostic of early primary syphilis. The prozone phenomenon causes false-negative VDRL in secondary syphilis with very high antibody titers — it is not the scenario here.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.