This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
A male patient with symptoms of urethritis. Examination reveals only pus cells without any organism. Most likely cause is: 
Select and explain. 
A. Chlamydia trachomatis, B. H. ducreyi, C. Treponema pallidum, D. M. tuberculi
A: Chlamydia trachomatis
Answer: A: Chlamydia trachomatis 
Explanation: Ans. is 'a' i.e., Chlamydia trachomatisRef: Harrison, 19th/e, p. 1165 and Ananthanarayan, 9th/e, p. 415Gram staining of samples shows presence of abundant pus cells but no bacteria (Intracellular bacteria) and Chlamydia cannot be grown on laboratory media like blood agar, chocolate agar, MacConkey agar etc.