A 29 year old sexually active male presents with burning micturition. On clinical examination no ulcer is seen in the genitals. Urine examination shows 50 WBCs/HPF, no RBC's, leucocyte esterase positive, gonococcal culture negative. What could be the MOST probable causative organism?

C. trachomatis is the most common cause of nongonococcal urethritis (NGU) and post gonococcal urethritis (PGU). NGU is diagnosed by documentation of a leukocytic urethral exudate and by exclusion of gonorrhea by Gram's staining or culture. Symptoms include urethral discharge (often whitish and mucoid rather than frankly purulent), dysuria, and urethral itching. Physical examination may reveal meatal erythema and tenderness as well as a urethral exudate. At least one-third of male patients with C. trachomatis urethral infection have no evident signs or symptoms of urethritis. Asymptomatic chlamydial urethritis has been demonstrated in 5-10% of sexually active male adolescents screened at school-based clinics or community centers. Such patients generally have pyuria (15 leukocytes per 400x microscopic field in the sediment of first-void urine), a positive leukocyte esterase test, or an increased number of leukocytes on a Gram-stained smear prepared from a urogenital swab. Ref: Gaydos C.A., Quinn T.C. (2012). Chapter 176. Chlamydial Infections. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds),Harrison's Principles of Internal Medicine, 18e.