**Core Concept**
The differential diagnosis of epididymo-orchitis and testicular torsion involves identifying clinical features that distinguish between these two conditions. Epididymo-orchitis is an inflammatory condition often caused by bacterial infection, whereas testicular torsion is a surgical emergency characterized by twisting of the spermatic cord. Accurate diagnosis is crucial to prevent testicular loss and ensure proper management.

**Why the Correct Answer is Right**
The presence of cremasteric reflex is a key differentiator between epididymo-orchitis and testicular torsion. The cremasteric reflex is a normal physiological response where contraction of the cremaster muscle occurs upon stroking the skin of the upper inner thigh. In testicular torsion, the cremasteric reflex is often absent or diminished due to ischemia and pain. In contrast, patients with epididymo-orchitis typically have a preserved cremasteric reflex.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while pain is a common feature of both conditions, it is not a reliable differentiator. Both epididymo-orchitis and testicular torsion can cause severe pain.

**Option B:** This option is incorrect because while fever is often present in epididymo-orchitis, it is not a reliable differentiator between the two conditions. Testicular torsion can also be associated with fever.

**Clinical Pearl / High-Yield Fact**
The cremasteric reflex is a crucial clinical sign to assess in the differential diagnosis of epididymo-orchitis and testicular torsion. A diminished or absent cremasteric reflex in a patient with acute scrotal pain should raise suspicion for testicular torsion, and prompt surgical evaluation is necessary.

**Correct Answer: C.** The cremasteric reflex is a key differentiator between epididymo-orchitis and testicular torsion.