- The correct answer is Primary syphilis.

- It typically presents as a solitary, painless ulcer (chancre) with a firm, raised border.
- This lesion appears at the site of Treponema pallidum entry and heals spontaneously within weeks.
- In contrast, chancroid ulcers are painful and soft, herpes lesions are multiple and painful vesicles or ulcers, and lymphogranuloma venereum initially causes a small, painless papule or ulcer but is often accompanied by painful inguinal lymphadenopathy.
- The painless chancre with a firm border is thus pathognomonic of primary syphilis.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Infectious Diseases Chapter