**Core Concept**
Mediastinal lymph node calcification is a hallmark radiological finding in sarcoidosis, a granulomatous disease characterized by non-caseating granulomas in the lungs and mediastinal lymph nodes. These calcifications result from chronic inflammation and granuloma formation, particularly in the hilar and mediastinal regions.

**Why the Correct Answer is Right**
In sarcoidosis, granulomas form in the mediastinal lymph nodes due to an immune-mediated reaction, often triggered by unknown antigens. Over time, these granulomas undergo calcification, especially in the early to middle stages of the disease. This is a classic radiological sign, frequently seen on chest X-rays or CT scans as "crazy-paving" or "tram-track" patterns. Calcification is more common in the hilar and upper mediastinal nodes and is pathognomonic for sarcoidosis in many cases.

**Why Each Wrong Option is Incorrect**
Option A: Metastatic neoplasms typically cause lymph node enlargement with soft tissue density, not calcification. Calcification in metastases is rare and usually occurs in specific cancers like breast or lung, but not as a typical feature.
Option B: Lymphoma presents with enlarged, non-calcified, often heterogeneous lymph nodes, and calcification is not a feature.
Option D: Bronchiectasis is associated with dilated airways, chronic infection, and mucus retention, not lymph node calcification.

**Clinical Pearl / High-Yield Fact**
Mediastinal lymph node calcification is most specifically associated with sarcoidosis and should prompt consideration of this condition in patients with bilateral hilar shadows, cough, or fatigue. It is a key diagnostic clue on imaging and often precedes clinical symptoms.

✓ Correct Answer: C. Sarcoidosis