## Primary Syphilis — Most Common Sites **Key Point:** The glans penis is the most common site of primary syphilis (chancre) in heterosexual males, accounting for approximately 70–80% of cases. ### Anatomical Distribution in Males | Site | Frequency | Notes | | --- | --- | --- | | Glans penis | 70–80% | Most common; painless indurated ulcer | | Prepuce | 10–15% | Second most common | | Shaft of penis | 5–10% | Less frequent | | Perianal region | 2–5% | Rare in heterosexual males; more common in MSM | **Clinical Pearl:** The chancre is classically described as a painless, indurated ulcer with a clean base, raised borders, and regional non-tender lymphadenopathy (Hunterian chancre). This constellation of findings is pathognomonic for primary syphilis. **High-Yield:** In women, the most common sites are the labia majora, labia minora, and cervix. In MSM, the perianal region and rectum are more frequently involved. **Warning:** Do not confuse the painless nature of the primary chancre with herpes simplex (which is painful) or haemophilus ducreyi (soft chancre, which is also painful). The painlessness is a key diagnostic feature. ### Darkfield Microscopy Confirmation The identification of motile, corkscrew-shaped *Treponema pallidum* on darkfield microscopy confirms the diagnosis and is the gold standard for primary syphilis diagnosis in the presence of a chancre. [cite:Park 26e Ch 7]
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