**Core Concept**
Varicocele is a condition characterized by the enlargement of the pampiniform plexus of veins within the scrotum, which can cause scrotal swelling. The swelling is typically non-reducible but may change in size depending on the patient's position.

**Why the Correct Answer is Right**
The described clinical scenario is consistent with a varicocele, which is a type of varicosity that occurs in the scrotal veins. As the patient lies down overnight, the swelling may decrease due to gravity, only to reappear as the patient stands up and increases venous pressure. This is because the pampiniform plexus of veins is subjected to increased pressure when the patient is standing, causing the veins to dilate and become more prominent. The **internal spermatic veins** are a key anatomical structure involved in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocele is a condition characterized by the accumulation of fluid in the tunica vaginalis, which can cause scrotal swelling. However, hydrocele typically does not change in size with position and is usually reducible.
**Option B:** Testicular torsion is a medical emergency characterized by the twisting of the spermatic cord, which can cause sudden scrotal swelling. However, testicular torsion does not typically change in size with position and is usually associated with severe pain.
**Option C:** Epididymitis is an inflammation of the epididymis, which can cause scrotal swelling. However, epididymitis typically does not change in size with position and is usually associated with pain, fever, and urinary symptoms.

**Clinical Pearl / High-Yield Fact**
Varicocele is a common cause of scrotal swelling and is often associated with infertility. The **"Val Sal" maneuver**, where the patient is asked to stand with the affected side down and the unaffected side up, can help differentiate varicocele from other causes of scrotal swelling.

**Correct Answer: C. Epididymitis.**