Final answer:
The most common cause of nongonococcal urethritis (NGU) is Chlamydia trachomatis, followed by pathogens like Mycoplasma genitalium. Though NGU's symptoms may be mild or nonexistent, untreated infections can cause more severe reproductive health issues.
Step-by-step explanation:
The most common cause of nongonococcal urethritis (NGU) is Chlamydia trachomatis. NGU refers to inflammation of the urethra that is not related to Neisseria gonorrhoeae, the pathogen responsible for gonorrhea. Mycoplasma genitalium is also a significant pathogen, responsible for a substantial number of NGU cases, particularly recurrent or persistent infections. Other species such as Ureaplasma urealyticum, and Mycoplasma hominis are less commonly involved but can cause infection as well.
Chlamydia infections, although often asymptomatic, are a major cause of NGU and can eventually lead to complications if untreated. Asymptomatic individuals may not seek treatment, which could result in the disease spreading to reproductive organs, potentially causing pelvic inflammatory disease (PID) in women and epididymitis and prostatitis in men. Comparatively, gonorrhea caused by Neisseria gonorrhoeae is another common STI which may present with painful urination and genital discharge. However, it's important to distinguish it from NGU as it requires different treatment, and resistance to antibiotics is a growing concern.