A 28-year-old man is evaluated in a clinic for anal pain and brownish-white rectal discharge that has steadily increased in amount and frequency over the past week. He denies fever, chills, urethral symptoms, bloody stool, genital ulcers or other lesions, and other systemic symptoms. He reports one new male sex partner with whom he engages in receptive anal intercourse, using condoms only intermittently. He is presumptively diagnosed with proctitis.