## Clinical Diagnosis The presentation of a painless, indurated ulcer with a clean base and dark-field microscopy showing motile spirochetes is pathognomonic for **primary syphilis** caused by *Treponema pallidum*. ## Drug of Choice **Key Point:** Penicillin G benzathine (2.4 million units IM as a single dose) is the first-line and gold-standard treatment for primary syphilis worldwide, including in India. ## Why Penicillin? | Feature | Penicillin G Benzathine | Alternatives | |---------|-------------------------|---------------| | **Efficacy** | 95–98% cure rate | Slightly lower in some settings | | **Mechanism** | β-lactam; inhibits cell wall synthesis | Same mechanism | | **Dosing** | Single IM injection | Oral or IV regimens | | **Cost** | Very low | Higher | | **Guideline Status** | WHO, CDC, NACO first-line | Second-line | | **Pregnancy** | Safe | Doxycycline contraindicated | **High-Yield:** *Treponema pallidum* is a gram-negative spirochete with a thin peptidoglycan layer highly susceptible to penicillin. No resistance has been documented. ## Alternative Regimens **Clinical Pearl:** In penicillin-allergic patients: - **Non-pregnant:** Doxycycline 100 mg PO BD × 14 days (primary syphilis) - **Pregnant:** Ceftriaxone 1 g IM daily × 10 days (β-lactam, safe in pregnancy) - **Azithromycin:** Emerging resistance; no longer recommended as first-line [cite:CDC STI Guidelines 2021] ## Mnemonic **PENICILLIN for Pallidum** — The organism name and drug start with the same letter; penicillin is the classic, irreplaceable choice.
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