**Core Concept**
Transillumination is a physical examination technique used to assess the contents of a scrotal swelling. It involves shining a light through the swelling in a dark room to see if it transilluminates, meaning the light passes through it. This test is used to differentiate between solid and fluid-filled masses.

**Why the Correct Answer is Right**
The correct answer is a hydrocele, which is a fluid accumulation in the tunica vaginalis of the testis. This fluid-filled space allows the light to pass through, resulting in transillumination. The tunica vaginalis is a serous membrane that surrounds the testis and epididymis. In a hydrocele, fluid accumulates in this space, causing the scrotum to swell.

**Why Each Wrong Option is Incorrect**
**Option A:** A varicocele is an abnormal enlargement of the pampiniform plexus, which is a network of veins that carries blood away from the testis. While it can cause a scrotal swelling, it does not typically transilluminate because it is a solid mass of veins.

**Option B:** A testicular tumor is a solid mass that arises from the testis itself. It would not transilluminate because it is a solid structure that blocks the passage of light.

**Option C:** An epididymitis is an inflammation of the epididymis, a tube-like structure that stores and carries sperm from the testis. While it can cause a scrotal swelling, it is usually accompanied by signs of inflammation such as pain and tenderness.

**Clinical Pearl / High-Yield Fact**
When performing a physical exam on a scrotal swelling, remember to use transillumination to differentiate between solid and fluid-filled masses. This can help guide further diagnostic testing and treatment.

**Correct Answer: A. Hydrocele**