Which sexually transmitted infection is indicated by a painless, reddish lesion on the penis?

The presence of a painless, reddish lesion on the penis is characteristic of syphilis, particularly in its primary stage. In this initial phase, a syphilitic sore, known as a chancre, appears at the site of infection. This sore is typically round, firm, and painless, making it an important indicator of syphilis infection.

The appearance of the lesion is significant because early detection and treatment of syphilis are crucial to preventing the progression to later stages, which can lead to more severe health complications. Therefore, identifying these symptoms correctly is vital for timely management.

In contrast, other sexually transmitted infections often present with different symptoms. For example, gonorrhea and chlamydia usually manifest with painful urination or discharge, while herpes simplex commonly results in painful blisters or sores.