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: Microbiology
[{'content': 'A man presents to a ST clinic with urethritis and urethral discharge. Gram stain shows numerous pus cells but no microorganism. The culture is negative on the routine laboratory media. The most likely agent is - \nSelect and explain. \nA. Chlamydia trachomatis, B. Haemophilus ducreyi, C. Treponema pallidum, D. Neisseria gonorrhoeae', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]