A 43-year-old woman seeks care for a malodorous yellow-green vaginal discharge. A wet mount of her vaginal secretions is examined, and motile trichomonads are seen. Urine nucleic acid amplification testing for Chlamydia trachomatis and Neisseria gonorrhoeae infections are performed, and both are negative. The diagnosis of trichomoniasis is made, and the plan is to treat her with oral multi-dose metronidazole. She reports she developed urticaria and facial edema when receiving metronidazole treatment for giardiasis 2 years prior; these symptoms resolved with diphenhydramine and oral corticosteroids.