Which of the following organisms is most likely to be implicated as a cause of urethritis that persists after antibiotic therapy for gonorrhea?

Chlamydia, Mycoplasma, and Ureaplasma are not effectively treated by penicillins and cephalosporins, and are impoant causes of post-gonococcal urethritis. Chlamydial urethritis can be diagnosed by using fluorescent antibodies to identify inclusions in epithelial cells. Actinomyces is a mouth commensal that rarely causes a deeper oral infection. Mycobacteria cause chronic granulomatous diseases such as tuberculosis and leprosy. Nocardia can cause necrotizing pneumonia and disseminated disease.