A 9-month-old boy is brought to the physician by his mother, who noticed that her son had developed scrotal swelling. Physical examination reveals a scrotal mass. The lesion can be transilluminated and is composed of clear serous fluid. What is the appropriate diagnosis?

**Core Concept**
The scrotal swelling in a 9-month-old boy is likely caused by a hydrocele, which is a collection of fluid in the scrotum. Hydroceles are often associated with patent processus vaginalis, a congenital anomaly where the peritoneal cavity communicates with the scrotum.

**Why the Correct Answer is Right**
The lesion can be transilluminated due to its fluid composition, which is a key diagnostic feature. Hydroceles contain clear serous fluid that accumulates in the scrotum, typically as a result of the communication between the peritoneal cavity and the scrotum. This fluid is produced by the peritoneal cavity and flows into the scrotum through the patent processus vaginalis.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Testicular torsion** would present with severe pain and swelling, but the fluid composition would not be clear serous fluid.
* **Option B:** **Epididymitis** would present with inflammation and swelling of the epididymis, but the fluid composition would be purulent, not clear serous fluid.
* **Option C:** **Spermatocele** is a benign cystic lesion of the epididymis, but it would not be transilluminable and would contain spermatozoa, not clear serous fluid.

**Clinical Pearl / High-Yield Fact**
Remember that hydroceles in infants and children are often associated with patent processus vaginalis, and treatment may involve surgical correction of the patent processus vaginalis.

**Correct Answer:** C. Hydrocele.

✓ Correct Answer: C. Hydrocele