A 28-year-old man from Delhi presents with a painless, indurated ulcer on the glans penis for 3 weeks. He reports a single episode of unprotected sexual contact 4 weeks ago. On examination, the ulcer has a clean base with rolled edges and regional lymphadenopathy (non-tender, rubbery nodes). RPR is negative. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis: Primary Syphilis The patient presents with the classic triad of primary syphilis: - **Painless, indurated ulcer** (chancre) with clean base and rolled edges - **Regional lymphadenopathy** (non-tender, rubbery — inguinal bubo) - **Appropriate epidemiology** (unprotected sexual contact 4 weeks prior) ### Why RPR Negativity Does NOT Exclude Primary Syphilis **Key Point:** In early primary syphilis (< 4–6 weeks), serological tests (RPR, VDRL) may be negative despite active infection. Treponemal antibodies develop over 3–6 weeks; non-treponemal antibodies appear even later. **High-Yield:** Seronegativity in the first 4–6 weeks of primary syphilis is well-documented and does NOT rule out the diagnosis when clinical features are classic. ### Diagnostic Approach in Seronegative Primary Syphilis | Investigation | Sensitivity in Early Primary | Timing | Clinical Use | |---|---|---|---| | **Dark-field microscopy** | 75–95% | Immediate, at bedside | Gold standard for early primary syphilis | | **Direct fluorescent antibody (DFA)** | 75–95% | Same visit | Alternative to dark-field; requires fluorescence microscope | | **RPR/VDRL** | 70–80% at 3–4 weeks | Delayed | Becomes positive later; not reliable in first 4–6 weeks | | **FTA-ABS / TP-PA** | 85–95% at 3–4 weeks | Delayed | Treponemal tests; become positive before non-treponemal | ### Correct Management Sequence 1. **Confirm diagnosis** with dark-field microscopy or DFA on fresh exudate from the ulcer 2. **Do NOT wait for serology** to become positive — this delays treatment 3. **Initiate penicillin** immediately upon microbiological confirmation 4. **Repeat serology** at 2 weeks (RPR/VDRL should seroconvert by then) **Clinical Pearl:** The presence of motile, corkscrew-shaped *Treponema pallidum* on dark-field microscopy is diagnostic and permits immediate treatment initiation without waiting for serology. ## Why the Correct Answer Is Dark-Field Microscopy / DFA **Mnemonic:** **SERONEGATIVE PRIMARY = MICROSCOPY FIRST** — When clinical features are classic but serology is negative, direct visualization of the organism is the diagnostic gold standard. [cite:Harrison 21e Ch 207]