Epididymitis: This is the most likely diagnosis due to the location of the swelling, which is often associated with inflammation of the epididymis, a tube at the back of the testicle that stores and carries sperm. The swelling is typically accompanied by pain and may be caused by a bacterial or viral infection.

Hydrocele: A hydrocele is an accumulation of fluid in the tunica vaginalis, a sac that surrounds the testicle, which can cause swelling in the lower part of the testicle. It is often painless and may be primary (idiopathic) or secondary to trauma, infection, or tumor.
Spermatocele: A spermatocele is a cystic structure that forms at the head of the epididymis and can cause swelling in the lower part of the testicle. It is usually painless and contains sperm.
Varicocele: A varicocele is an enlargement of the veins within the scrotum, similar to varicose veins, which can cause swelling and a feeling of heaviness in the lower part of the testicle.

Testicular Torsion: Although less common, testicular torsion is a medical emergency that requires immediate attention. It occurs when the spermatic cord becomes twisted, cutting off the blood supply to the testicle. Swelling and severe pain are typical symptoms.
Testicular Cancer: While rare, testicular cancer can present with a painless swelling or lump in the testicle. It is crucial to examine and investigate any testicular swelling to rule out this potentially life-threatening condition.

Epididymal Cyst: These are rare, benign cysts that can form in the epididymis and cause swelling in the lower part of the testicle.
Adrenal Rest Tumors: These are rare, benign tumors that can occur in the testicle and cause swelling. They are often associated with congenital adrenal hyperplasia.
Filariasis: In endemic areas, filariasis (caused by parasitic worms) can lead to hydrocele and swelling of the scrotum, including the lower part of the testicle.