**Core Concept**
Testicular torsion and epididymo-orchitis are two conditions that present with acute scrotal pain, but they require prompt differentiation due to their distinct management strategies. Testicular torsion is a surgical emergency that necessitates immediate intervention to salvage the affected testis, whereas epididymo-orchitis is typically treated with antibiotics.

**Why the Correct Answer is Right**
Color Doppler ultrasound is the most accurate and reliable method to differentiate between testicular torsion and epididymo-orchitis. It assesses blood flow to the testis by detecting the presence or absence of spectral Doppler signals. In testicular torsion, blood flow is significantly reduced or absent due to the twisted spermatic cord, whereas in epididymo-orchitis, blood flow is typically preserved. This distinction allows for timely diagnosis and appropriate management.

**Why Each Wrong Option is Incorrect**
**Option A:** The "Angel sign" is not a recognized or reliable clinical or radiological sign for differentiating between testicular torsion and epididymo-orchitis.

**Option B:** The Prehn sign, which involves elevating the affected testis to assess pain relief, is not a reliable method for differentiating between these two conditions. It can be positive in both testicular torsion and epididymo-orchitis, making it a less useful diagnostic tool.

**Option D:** While MRI can be used to evaluate the scrotum, it is not the best initial imaging modality for differentiating between testicular torsion and epididymo-orchitis due to its higher cost, limited availability, and longer scanning time compared to ultrasound.

**Clinical Pearl / High-Yield Fact**
In cases of suspected testicular torsion, prompt imaging with Color Doppler ultrasound is essential to avoid unnecessary delay in surgical intervention. A negative Doppler study (absence of blood flow) is highly suggestive of testicular torsion, warranting immediate surgical exploration.

✓ Correct Answer: C. Color Doppler