What is the most appropriate treatment for a painful ulcer on the labia diagnosed during a clinic visit?

The most appropriate treatment for a painful ulcer on the labia diagnosed during a clinic visit is Valaciclovir 1 g PO twice daily for 7 days. This choice is correct because a painful ulcer in this area is likely symptomatic of a herpes simplex virus (HSV) infection.

Valaciclovir is an antiviral medication specifically effective against HSV, and it helps to reduce the duration and severity of symptoms during an outbreak. It works by inhibiting viral replication, which can alleviate pain, promote healing, and reduce the risk of transmission to sexual partners.

The options involving azithromycin, doxycycline, and penicillin G benzathine are more commonly indicated for bacterial infections, such as chlamydia, gonorrhea, or syphilis, rather than viral infections like herpes. Given the presentation of a painful ulcer, the clinical suspicion would lean more towards a herpes infection, making the use of an antiviral like valaciclovir the most suitable treatment.