This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Which of the following organisms is most likely to be implicated as a cause of urethritis that persists after antibiotic therapy for gonorrhea? 
Select and explain. 
A. Actinomyces, B. Chlamydia, C. Mycobacteria, D. Nocardia
B: Chlamydia
Answer: B: Chlamydia 
Explanation: 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.