**Core Concept**
Sarcoidosis is a systemic granulomatous disease characterized by the formation of non-caseating granulomas in various organs, most commonly the lungs and lymph nodes. The increased level of Angiotensin-Converting Enzyme (ACE) is a common finding in sarcoidosis, although not specific to the disease.

**Why the Correct Answer is Right**
The patient's presentation of prolonged fever and cough, combined with mediastinal widening on chest X-ray, suggests involvement of the lymph nodes, which is a common manifestation of sarcoidosis. The increased level of ACE levels supports this diagnosis, as elevated ACE levels are often found in patients with sarcoidosis. ACE is produced by the epithelioid cells within the granulomas in sarcoidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Tuberculosis (TB) typically presents with chronic cough, fever, and weight loss, but it usually does not cause mediastinal widening or elevated ACE levels. TB is also more common in immunocompromised individuals or those with a history of exposure to TB.
**Option B:** Lymphoma can cause mediastinal widening and systemic symptoms, but it is not typically associated with elevated ACE levels. Lymphomas can be distinguished from sarcoidosis by their specific histological features and the presence of systemic symptoms.
**Option C:** Histoplasmosis is a fungal infection that can cause mediastinal widening and systemic symptoms, but it is not typically associated with elevated ACE levels. Histoplasmosis can be distinguished from sarcoidosis by its specific histological features and the presence of systemic symptoms.
**Option D:** Idiopathic pulmonary fibrosis (IPF) can cause chronic cough and systemic symptoms, but it is not typically associated with mediastinal widening or elevated ACE levels. IPF can be distinguished from sarcoidosis by its specific histological features and the presence of systemic symptoms.

**Clinical Pearl / High-Yield Fact**
Sarcoidosis is a systemic disease that can affect multiple organs, including the lungs, lymph nodes, skin, eyes, and heart. The diagnosis of sarcoidosis is based on a combination of clinical findings, laboratory tests, and histological examination of tissue samples.

**Correct Answer: C. Sarcoidosis**