**Core Concept:**
Epididymo-orchitis and torsion testis are two important clinical conditions affecting the testis. Epididymo-orchitis is an inflammation of the epididymis and the testis, usually caused by bacterial infections. Torsion testis is a condition where the testis twists on its pedicle, leading to reduced blood flow and subsequent tissue damage. Correct differentiation between these two conditions is essential for timely and appropriate management.

**Why the Correct Answer is Right:**
The correct answer to differentiate between epididymo-orchitis and torsion testis is **option C**: **Physical examination and testicular ultrasound**.

Physical examination plays a crucial role in diagnosing these conditions. In epididymo-orchitis, the testis is usually tender, swollen, and firm. The patient may complain of dull pain, which is usually worse at night and relieved by elevation. In torsion testis, the testis is tender, swollen, and firm, and the patient typically presents with acute severe pain, which worsens with movement and is relieved by down-elevation.

**Why Each Wrong Option is Incorrect:**

**Option A (History and examination)**: While history and examination are essential for diagnosis, they are not specific enough to differentiate between epididymo-orchitis and torsion testis solely based on these factors.

**Option B (Lab tests)**: Laboratory tests like blood and urine cultures are performed to identify a possible infectious cause for epididymo-orchitis. However, they do not differentiate epididymo-orchitis from torsion testis.

**Option D (Testicular Doppler ultrasound)**: Testicular Doppler ultrasound can be helpful in assessing blood flow and diagnosis of testicular torsion. However, it is not sufficient to differentiate epididymo-orchitis and torsion testis due to the similarity in presentation and clinical features.

**Clinical Pearl:**

In clinical practice, a combination of detailed history, examination, and judicious use of testicular ultrasound can help differentiate between epididymo-orchitis and torsion testis. However, it is essential to consider that the initial presentation of both conditions is similar.

**Correct Answer Explanation:**

A combination of a thorough history, clinical examination, and testicular ultrasound can help differentiate between epididymo-orchitis and torsion testis. However, it is important to remember that both conditions share similar symptoms and signs at the initial presentation.