What characterizes the primary stage of syphilis?

The primary stage of syphilis is characterized by the presence of firm, painless ulcers known as chancre. These sores typically develop at the site of the Treponema pallidum infection, which is often the genital region, anus, or mouth. The chancre appears about three weeks after exposure to the bacteria and can range in size but is usually round, indurated, and ulcerated. The lesion is usually asymptomatic, aside from the visible sore, and does not cause pain, which is a distinctive feature compared to other ulcerative lesions in different conditions.

In this stage, the regional lymph nodes may become enlarged and firm due to localized infection, but they are not typically massively enlarged compared to later stages or other infections. Other symptoms that are seen in different stages of syphilis or in other conditions may include rashes or painful lesions, but these do not characterize the primary stage. Therefore, the presence of firm, painless ulcers is a hallmark of primary syphilis, confirming the selection of this answer.