A 34-year-old woman who has sex with men presents with 1-week onset of anorectal pain, tenesmus, and purulent rectal discharge. She denies fever, chills, diarrhea, hematochezia, or other symptoms. She reports receptive vaginal and anal intercourse with one sex partner in the past 6 months. Both she and her partner had a negative HIV test 4 months ago. They do not use condoms. Physical examination is normal except for tenderness on rectal examination. Anoscopy shows mild erythema and gross purulence, but no ulcers or bloody discharge. A presumptive diagnosis of proctitis is made. She weighs about 65 kg.