A patient presents with a painless vulvar ulcer following her boyfriend's treatment for a sexually transmitted infection. What is the most likely diagnosis?

In this clinical scenario, the presence of a painless vulvar ulcer suggests a likely diagnosis of syphilis. The primary stage of syphilis is characterized by the appearance of a chancre, which is a firm, painless ulcer commonly found on the genitals. This feature is distinct from ulcers associated with other sexually transmitted infections.

Given that the patient presented with the ulcer following her boyfriend's treatment for a sexually transmitted infection, it is pertinent to consider that syphilis is commonly asymptomatic in its early stages and can be transmitted even if the infected person does not exhibit symptoms.

Additionally, the other options present different characteristics and are associated with other clinical features. For instance, chancroid typically presents as a painful genital ulcer, while herpes often manifests with painful lesions that may be associated with systemic symptoms like fever or lymphadenopathy. Granuloma inguinale usually leads to a progressive, beefy red ulcer, and can also become indolent but is notably more destructive and less common in presentation than syphilis.

Therefore, given the combination of a painless ulcer and the context of exposure through a partner being treated for a sexually transmitted infection, the most likely diagnosis is indeed syphilis.