This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
An 18-year-old male patient has been seen in the clinic for urethral discharge. He is treated with IM ceftriaxone, but the discharge has not resolved and the culture has returned as no growth. Which of the following is the most likely etiologic agent to cause this infection? 
Select and explain. 
A. Ceftriaxone-resistant gonococci, B. Chlamydia psittaci, C. Chlamydia trachomatis, D. Herpes simplex virus
C: Chlamydia trachomatis
Answer: C: Chlamydia trachomatis 
Explanation: About half of all cases of nongonococcal urethritis are caused by C trachomatis. Urea plasma urealyticum and Trichomonas vaginalis are rarer causes of urethritis. Herpes simplex would present with vesicular lesions and pain, not with a meatal discharge. Chlamydia psittaci is the etiologic agent in psittacosis. Almost all gonococci are susceptible to ceftriaxone at recommended doses.