Final answer:
A painless swelling on the scrotum that transilluminates on ultrasound can be caused by testicular cancer or epididymitis.
Step-by-step explanation:
A painless swelling on the scrotum that transilluminates on ultrasound can be caused by several conditions, including testicular cancer and epididymitis. Testicular cancer is the most common cancer in males between the ages of 20 and 39, and a lump or swelling in one testis is a possible sign. Epididymitis, on the other hand, is inflammation of the epididymis and can cause discomfort, pain, and swelling in the scrotum.