A 28-year-old man from Mumbai presents to the STI clinic with a painless ulcer on the glans penis for 3 weeks. He reports a single sexual contact 4 weeks ago. On examination, the ulcer has a well-demarcated border, indurated base, and is non-purulent. Regional lymph nodes are enlarged, firm, and non-tender. Serum RPR is positive (1:16) and FTA-ABS is positive. Dark-field microscopy of ulcer exudate shows motile, spiral organisms. What is the most appropriate next step in management?

Explanation

## Diagnosis: Primary Syphilis The clinical presentation is classic for primary syphilis: - **Painless indurated ulcer (chancre)** with well-demarcated borders and firm base - **Unilateral regional lymphadenopathy** (non-tender, firm — "rubbery") - **Positive serology** (RPR and FTA-ABS both reactive) - **Dark-field microscopy** confirms *Treponema pallidum* (motile, thin, corkscrew-shaped spirochetes) ## Treatment of Primary Syphilis **Key Point:** Benzathine penicillin G is the first-line agent for all stages of syphilis in non-pregnant patients. | Stage | First-Line Therapy | Dose | Duration | |-------|-------------------|------|----------| | Primary, Secondary, Early Latent | Benzathine PCN G | 2.4 MU IM single dose | Once | | Late Latent / Tertiary | Benzathine PCN G | 2.4 MU IM weekly | 3 weeks | | Neurosyphilis | Aqueous PCN G IV | 18–24 MU daily | 10–14 days | | Penicillin allergy (non-pregnant) | Doxycycline | 100 mg BD | 28 days | **High-Yield:** For primary syphilis, a **single IM injection** of benzathine penicillin G 2.4 million units is curative in >95% of cases. **Clinical Pearl:** The Jarisch-Herxheimer reaction (fever, chills, myalgia, hypotension) may occur 6–12 hours after the first penicillin dose due to release of endotoxin from dying spirochetes. Counsel the patient and manage supportively with antipyretics and fluids. ## Why Benzathine Penicillin? 1. **Highest cure rate** for primary syphilis (>95%) 2. **Excellent tissue penetration** (though CSF penetration is poor — not suitable for neurosyphilis) 3. **Long half-life** ensures adequate serum and tissue levels for 2–3 weeks 4. **Cost-effective** and widely available 5. **WHO and CDC guideline standard** **Mnemonic:** **PENICILLIN FIRST** — **P**rimary syphilis → **E**arly treatment → **N**eed **I**M **C**ure → **I**njection **L**ong-acting → **L**ow relapse → **I**ncreased **N** survival