A young woman presents with a painless vulvar ulcer after a partner was treated for an STI. What is the likely diagnosis?

The presentation of a painless vulvar ulcer in the context of a recent partner treated for a sexually transmitted infection strongly suggests a diagnosis of syphilis. Specifically, the primary stage of syphilis is characterized by the development of a chancre, which is typically a painless ulceration. This matches the symptom described in the question.

Additionally, the timing of the onset of the ulcer after the partner's treatment for an STI supports a syphilis diagnosis, as syphilis can be asymptomatic and easily transmitted to partners. The other potential diagnoses either typically present with painful lesions or are associated with other defining characteristics that would not fit this scenario as well as syphilis.

Herpes, for instance, often presents with painful vesicular lesions, and chancroid characteristically causes painful ulcers as well. Granuloma inguinale usually presents as a raised, beefy-red ulcer rather than a simple ulcer and is also not typically associated with the history of a partner testing positive for an STI in this manner. These details make syphilis the most likely diagnosis in this case.