A sexually active 20-year-old woman presents with painful vesicular lesions on the vulva. What is the most likely diagnosis?

The presentation of painful vesicular lesions on the vulva in a sexually active 20-year-old woman strongly suggests genital herpes, which is caused by the herpes simplex virus (HSV). Genital herpes is characterized by the appearance of multiple vesicles that can rupture, leading to painful ulcers. This is typical of the initial outbreak of HSV, which may also be accompanied by systemic symptoms such as fever, lymphadenopathy, and malaise.

Genital herpes is also notable for its recurrent episodes, as the virus remains dormant in the nerve ganglia and can re-emerge due to various triggers. The painful nature of the lesions distinguishes this condition from other types of vulvar lesions, where pain may not be prominent or is absent.

In contrast, candidal vaginitis typically presents with itching, discharge, and discomfort, but does not present with vesicular lesions. Condylomata acuminata (genital warts) caused by human papillomavirus may cause soft, fleshy growths but are not vesicular or painful in the same way. Syphilis can cause genital lesions but is generally characterized by painless ulcers (chancres) during the primary stage and systemic manifestations that are not usually presented acutely in the scenario described.