A 25-yeаr-оld grаduаte student presents with cоmplaining оf scrotal pain, which has been increasing over the past 2 days. He is sexually active and has had unprotected intercourse with multiple partners in the past couple of weeks. On examination, the mild swelling of the scrotum on the right is tender with palpation and the vas deferens is inflamed. What is the most likely diagnosis?