Epididymitis: This is an inflammation of the epididymis, a tube at the back of the testicle that stores and carries sperm. It's a common cause of testicular pain and swelling, especially in adolescents and young adults, often due to bacterial infection.

Testicular Torsion: Although more of an emergency, this condition involves the spermatic cord becoming twisted, cutting off the blood supply to the testicle. It's less common but can present with sudden, severe pain and swelling.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. It can cause a feeling of heaviness or aching in the testicle and is more common on the left side.
Hydrocele: A fluid-filled sac that can cause swelling of the testicle, which might be painful. It's more common in middle-aged men but can occur in adolescents.

Testicular Cancer: Although rare, especially in adolescents, testicular cancer can present as a painless testicular mass. However, some types can cause pain, making it crucial not to miss this diagnosis.
Appendix Testis Torsion (Hydatid of Morgagni): A small appendage at the top of the testicle can twist, causing severe pain. It's less common than testicular torsion but requires prompt attention.

Spermatocele: A cystic structure that forms on the epididymis and can cause discomfort or pain, though it's usually painless.
Cystic Dysplasia of the Testis: A rare congenital anomaly that can cause testicular masses or cysts, potentially painful.
Henoch-Schönlein Purpura (HSP): A form of vasculitis that can cause testicular pain and swelling, among other symptoms like rash and joint pain. It's more common in children but can occur in adolescents.