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: Medicine
The commonest cause of non-gonococcal urethritis is - 
Select and explain. 
A. Chlamydia, B. Ureaplasma urealyticum, C. E. coli, D. Proteus
A: Chlamydia
Answer: A: Chlamydia 
Explanation: None