A 28-year-old man presents to the STI clinic with a painless ulcer on the glans penis for 3 weeks. On examination, the ulcer has a clean base, indurated edges, and regional lymphadenopathy is present. He is otherwise asymptomatic. RPR is positive (1:16) and FTA-ABS is positive. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis & Stage Identification **Key Point:** This patient presents with classic features of **primary syphilis**: painless indurated ulcer (chancre) with clean base, indurated borders, and regional lymphadenopathy. Positive serology (RPR 1:16 and FTA-ABS) confirms treponemal infection. The diagnosis is established — no further diagnostic workup is needed before treatment. ## Management of Primary Syphilis — First-Line Treatment **High-Yield (CDC 2021 / WHO Guidelines):** The standard first-line treatment for **primary, secondary, and early latent syphilis** is: > **Benzathine penicillin G 2.4 million units IM as a SINGLE dose** The regimen of **weekly × 3 doses** (total 7.2 MU) is reserved for **late latent syphilis, latent syphilis of unknown duration, and tertiary syphilis (non-neurosyphilis)**. Administering 3 weekly doses for primary syphilis is **incorrect and over-treatment** per current guidelines. **Doxycycline 100 mg BD × 14 days** is the recommended alternative for penicillin-allergic patients with primary/secondary/early latent syphilis. Since no penicillin allergy is documented in this patient, doxycycline is not the first choice. ## Why Not the Other Options? | Option | Reasoning | |--------|-----------| | **A — Dark-field microscopy** | Useful for diagnosis in seronegative early primary syphilis; here serology is already positive, diagnosis is confirmed, and treatment should not be delayed | | **B — Lumbar puncture** | Reserved for suspected neurosyphilis (neurological/ocular symptoms, treatment failure, HIV with low CD4); not indicated in uncomplicated primary syphilis | | **C — Benzathine PCN G weekly × 3 weeks** | This regimen is for late latent / tertiary syphilis, NOT primary syphilis — over-treatment for this stage | | **D — Doxycycline 100 mg BD × 14 days** ✓ | Correct alternative for penicillin-allergic patients; however, since no allergy is stated, benzathine PCN G single dose would be ideal — BUT among the options given, option C specifies the WRONG regimen (3 weekly doses), making doxycycline the only correctly dosed treatment option available | **Clinical Pearl:** Per CDC STI Treatment Guidelines 2021 and KD Tripathi Essentials of Medical Pharmacology: Primary syphilis = Benzathine PCN G 2.4 MU IM **single dose**. The 3-week weekly regimen is explicitly for late latent disease. Since option C states "weekly for 3 weeks," it represents the wrong regimen for primary syphilis, making **doxycycline 100 mg BD × 14 days (option D)** the most appropriate choice among the given options. **Mnemonic: PRIMARY = SINGLE SHOT** — Primary syphilis gets a single 2.4 MU IM dose; THREE shots are for LATE/LATENT disease.