Final answer:
Chlamydia, caused by the bacterium Chlamydia trachomatis, is the most common STI in men, being particularly prevalent among young adults aged 15 to 24. It often does not show symptoms, leading to underdiagnosis and potential reproductive system complications if not treated.
Step-by-step explanation:
The most common STI in men is chlamydia, caused by the bacterium Chlamydia trachomatis. Chlamydia is renowned for being the most prevalent bacterial STI in the United States and often goes undiagnosed due to its asymptomatic nature in many individuals. Although it is more common in females, as reported by the Centers for Disease Control and Prevention (CDC), males can still contract chlamydia and experience complications such as nongonococcal urethritis (NGU), epididymitis, and potentially lead to more severe reproductive issues if left untreated.