Epididymitis: This is the most common cause of testicular pain in young men, often resulting from a bacterial infection, typically Chlamydia or gonorrhea. The symptoms include pain, swelling, and tenderness of the epididymis, which can be accompanied by urinary frequency and discharge.

Testicular torsion: Although less common than epididymitis, testicular torsion is a significant cause of acute testicular pain. It occurs when the spermatic cord becomes twisted, cutting off the blood supply to the testicle. Symptoms include sudden, severe pain, swelling, and elevation of the testicle.
Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can present with pain, swelling, and tenderness of the testicles.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. While often asymptomatic, it can cause a dull ache or discomfort in the testicle.

Testicular torsion (repeated for emphasis): Due to its potential for severe and irreversible damage if not promptly treated, testicular torsion must be considered an urgent diagnosis to rule out.
Testicular cancer: Although rare, testicular cancer can present with pain, a testicular mass, or swelling. Early detection is crucial for effective treatment.
Appendicitis (with referred pain): In some cases, appendicitis can refer pain to the testicles, especially if the appendix is located near the testicles.

Henoch-Schönlein purpura (HSP): A form of vasculitis that can cause testicular pain, swelling, and skin rash. It's more common in children but can occur in young adults.
Spermatocele: A cystic structure filled with sperm, which can cause discomfort or pain if it becomes large enough.
Scrotal hematoma or hemorrhage: Bleeding within the scrotum, which can be due to trauma or spontaneous causes, leading to pain and swelling.