Final answer:
A painless, clean base chancre should make you think of primary syphilis, an infectious disease caused by Treponema pallidum.
Step-by-step explanation:
A painless, clean base chancre is indicative of primary syphilis, an infectious disease caused by the bacterium Treponema pallidum. This primary lesion, known as a hard chancre, usually presents as a single ulcer on the genitals, cervix, or anus within 10 to 90 days after the transmission of the disease.
Unlike chancroid lesions caused by Haemophilus ducreyi, which are painful and may produce a highly contagious fluid, a hard chancre associated with primary syphilis is generally hard, painless, and, without treatment, will typically heal on its own within two to six weeks.
The presence of a hard chancre, along with the understanding of the sexual history and other clinical findings, aids in the diagnosis of primary syphilis.