Which of the following is a characteristic of a primary lesion of syphilis?

A primary lesion of syphilis is characterized by the presence of a chancre. This is a painless ulcer that typically appears at the site of infection, usually on the genitalia, anus, or mouth. The chancre is the first sign of syphilis during the primary stage of the disease and is crucial for diagnosis, as it is an identifiable marker of early infection. Understanding the significance of the chancre is important because it indicates the entry point of the Treponema pallidum bacterium, which is responsible for syphilis.

In contrast, other choices such as pustules, ulcers, and carbuncles do not specifically represent the initial stage of syphilis. Pustules are typically associated with infections like acne or chickenpox. Ulcers can be a general term for any break in the skin or mucous membrane and can result from various causes, not solely syphilis. Carbuncles refer to a cluster of boils, usually related to bacterial skin infections, which also do not pertain to syphilis. Thus, the chancre is the defining lesion for identifying the primary stage of this sexually transmitted infection.