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
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 - 
Select and explain. 
A. Chlamydia trachomatis, B. Haemophilus ducreyi, C. Treponema pallidum, D. Neisseria gonorrhoeae
A: Chlamydia trachomatis
Answer: A: Chlamydia trachomatis 
Explanation: None