A 19-year-old man is seen in clinic with a nonpurulent urethral discharge. In the last 6 weeks, he had sex with two female partners and reports that he rarely used condoms. A nucleic acid amplification test (NAAT) is ordered for Chlamydia trachomatis and Neisseria gonorrhoeae on a first-catch urine. He is empirically treated with oral doxycycline 100 mg twice daily for 7 days. His urethral symptoms do not improve, and he is seen again 5 days later. The tests for chlamydia and gonorrhea at the initial evaluation were negative. Additional testing is performed at this visit and a urine NAAT is positive for Trichomonas vaginalis.