- During the primary stage of syphilis, the infection typically presents as a painless ulcerative lesion known as a chancre.
- This lesion usually appears at the site of Treponema pallidum inoculation, such as the genitalia, anus, or mouth.
- The chancre is classically firm, round, and indurated with raised edges.
- It often goes unnoticed by the patient because it is painless and resolves spontaneously within 3 to 6 weeks without treatment.

In contrast:
- Chancroid (Option 1) presents with one or more painful, soft ulcers with ragged edges and base, often accompanied by painful inguinal lymphadenopathy.
- Herpes simplex virus infection (Option 2) causes painful vesicles and ulcers that occur in clusters and are associated with systemic symptoms such as fever and lymphadenopathy.
- Lymphogranuloma venereum (Option 4) initially presents as a small, often painless papule or ulcer but is followed by painful inguinal lymphadenopathy (bubo formation).

Thus, the painless ulcer typical of primary syphilis helps differentiate it from other sexually transmitted infections characterized by painful lesions.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Chapter 228: Syphilis and Other Treponemal Infections