Final answer:
Sexually transmitted bacteria are the most common cause of acute bacterial epididymitis in individuals under 35 years old.
Step-by-step explanation:
The most common cause of acute bacterial epididymitis in individuals under 35 years old is sexually transmitted bacteria. Bacteria in the urethra can back-flow through the urinary and reproductive structures to the epididymis, leading to infection and inflammation. In addition to pain and swelling, other symptoms of acute epididymitis include redness, warmth in the scrotum, and a fever.