Epididymitis: This is the most common cause of acute testicular pain and swelling in adults, often resulting from bacterial infection, typically affecting the epididymis, which is located at the back of the testicle.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections, which can lead to testicular tenderness and swelling.
Testicular torsion (early stages): Although more commonly associated with sudden, severe pain, early stages might present with tenderness and swelling before the full onset of symptoms.
Hydrocele: A condition characterized by fluid accumulation around a testicle, leading to swelling, which can sometimes be accompanied by tenderness.

Testicular Torsion: A medical emergency that requires immediate attention. It involves the twisting of the spermatic cord, cutting off the blood supply to the testicle. Prompt diagnosis and treatment are crucial to save the affected testicle.
Testicular Cancer: Although less common, testicular cancer can present with a painless testicular mass, but some patients may experience tenderness or discomfort. Early detection is key for effective treatment.
Appendix Testis Torsion (Hydatid of Morgagni): A small, vestigial appendage that can twist and cause pain, often mimicking testicular torsion or epididymitis.

Spermatocele: A cystic structure filled with sperm, usually located at the head of the epididymis, which can cause swelling but is typically painless.
Varicocele: Enlargement of the veins within the scrotum, similar to varicose veins, which can cause swelling and a feeling of heaviness, but is usually not painful.
Henoch-Schönlein Purpura (HSP): A form of vasculitis that can affect various parts of the body, including the testicles, leading to swelling and pain, though this is a rare presentation.