Final answer:
A man exposed to an STD is most at risk for epididymitis, an infection leading to pain and swelling in the scrotum, which requires antibiotic treatment to prevent irreversible damage and infertility.
Step-by-step explanation:
A man who has been exposed to a sexually transmitted disease (STD) is most at risk for epididymitis, which is an inflammation of the epididymis typically caused by a bacterial infection that may stem from an STD. This condition is characterized by pain, swelling, and redness in the scrotum, testicles, and groin area, as well as the potential for fever and urethral discharge. Prompt treatment of epididymitis with antibiotics and anti-inflammatory medication is crucial to prevent possible complications such as permanent damage to the reproductive organs and infertility.