Epididymitis: This is an inflammation of the epididymis, a tube at the back of the testicle that stores and carries sperm. It is often caused by a bacterial infection, commonly chlamydia or gonorrhea, which are sexually transmitted infections (STIs) prevalent in young adults. The symptoms include pain, swelling, and redness of the testicle, which aligns with the presentation of a swollen testicle.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can occur alone or in conjunction with epididymitis (epididymo-orchitis), and its symptoms include testicular pain and swelling.
Testicular Torsion: A condition where the spermatic cord (which contains the vas deferens, blood vessels, and nerves) becomes twisted, cutting off the blood supply to the testicle. It is a medical emergency that requires immediate attention. Symptoms include sudden, severe testicular pain, swelling, and abdominal pain.
Hydrocele: An accumulation of fluid in the tunica vaginalis, a potential space around the testicle, leading to swelling of the scrotum. It can be primary (idiopathic) or secondary due to inflammation, trauma, or tumors.

Testicular Cancer: Although less common, testicular cancer can present with a painless testicular swelling or a hard testicular mass. It is crucial to examine the testicles and consider ultrasound imaging to rule out cancer, especially if the swelling is painless or if there's a palpable mass.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. While usually asymptomatic and not immediately dangerous, a new varicocele, especially if it's painful or atypical in presentation, could be a sign of a blockage or other issue that needs medical evaluation.

Spermatocele: A cystic structure that forms at the head of the epididymis and contains sperm. It's usually painless and discovered incidentally but can cause discomfort or swelling if large enough.
Henoch-Schönlein Purpura (HSP): A form of vasculitis that affects blood vessels and can cause a variety of symptoms, including scrotal swelling and pain. It's more common in children but can occur in adults.
Filarial Infection: In areas where filariasis is endemic, infections like Wuchereria bancrofti can cause lymphatic obstruction leading to scrotal swelling (hydrocele) and other symptoms. This would be rare in a non-endemic area without travel history.