**Core Concept**
Infectious diseases that involve the testis but spare the epididymis are typically those that cause suppurative epididymo-orchitis but have a predilection for the testicular parenchyma. This can occur due to various bacterial pathogens that ascend through the vas deferens.

**Why the Correct Answer is Right**
The correct answer is **Tuberculous epididymo-orchitis**. Tuberculosis (TB) of the testis is a well-documented entity, where the testis is involved but the epididymis is relatively spared. This is because TB usually enters the testis through lymphatic or hematogenous spread, rather than ascending from the epididymis. The testicular parenchyma is more susceptible to TB infection, leading to granulomatous inflammation and caseation necrosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Echinococcal orchitis typically involves the epididymis and testis, with the formation of cysts and calcifications.
* **Option B:** Fungal orchitis, such as that caused by Candida, typically involves both the epididymis and testis.
* **Option C:** Bacterial epididymo-orchitis, such as that caused by E. coli, typically involves both the epididymis and testis.

**Clinical Pearl / High-Yield Fact**
Tuberculous epididymo-orchitis often presents with a subacute onset of testicular pain, swelling, and tenderness, which may be accompanied by systemic symptoms such as fever, weight loss, and night sweats.

**Correct Answer:** C. Tuberculous epididymo-orchitis.