A 28-year-old man presents with a painless indurated ulcer on the glans penis for 2 weeks. Dark-field microscopy is negative. VDRL is non-reactive. FTA-ABS (IgM) is positive. The most likely interpretation is:

• A Herpes simplex — DFM and VDRL are falsely negative
• B Chancroid — dark-field microscopy cannot detect Haemophilus ducreyi
• C Lymphogranuloma venereum — Chlamydia trachomatis cannot be detected by DFM
• D Primary syphilis — VDRL may be non-reactive in early lesions; treponemal test (FTA-ABS) is more sensitive ✓

Explanation

In primary syphilis, the VDRL (non-treponemal test) is non-reactive in up to 25-30% of cases during the chancre stage because IgM seroconversion precedes IgG, and VDRL mainly detects IgG. FTA-ABS becomes positive earliest (often within 2 weeks of chancre) and is the most sensitive test in primary syphilis. A positive FTA-ABS IgM with a painless indurated ulcer strongly supports primary syphilis. Repeat dark-field microscopy or PCR for T. pallidum should confirm.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.