A 23-year-old man presents with a painless right-sided scrotal mass that transilluminates. What is the most likely diagnosis?

The most likely diagnosis in this case is a hydrocele. A hydrocele presents as a fluid-filled sac around the testicle, which commonly causes a painless scrotal swelling. The characteristic feature of a hydrocele is that it transilluminates, meaning that when a light is placed behind the scrotum, the fluid in the hydrocele allows light to pass through, making it appear illuminated. This happens because the hydrocele is typically filled with clear fluid.

In contrast, while a spermatocele also presents as a painless scrotal mass, it usually contains sperm and may not always transilluminate as well as a hydrocele. Testicular cancer, which is generally associated with a solid mass, typically does not transilluminate and is often painful or presents with other systemic symptoms. A varicocele, caused by dilated veins within the scrotum, often feels like a "bag of worms" and does not transilluminate.

Thus, the presentation of a painless scrotal mass that transilluminates strongly points towards a hydrocele being the most likely diagnosis.