**Question:** Which of the following is associated with hypersensitivity pneumonitis -

A. Allergic bronchopulmonary aspergillosis
B. Drug-induced lung disease
C. Sarcoidosis
D. Pneumoconiosis

**Core Concept:**
Hypersensitivity pneumonitis (HP) is a type of interstitial lung disease (ILD) characterized by a delayed immune response to inhaled antigens, leading to inflammation and fibrosis of the lung parenchyma. This condition is classified into three types: acute, subacute, and chronic.

**Why the Correct Answer is Right:**

C. Sarcoidosis is a common granulomatous interstitial lung disease of unknown cause, characterized by the presence of non-caseating granulomas in the lungs. Although both HP and sarcoidosis can lead to interstitial lung disease, they have distinct clinical, radiological, and pathological features. In contrast to HP, sarcoidosis typically presents with bilateral hilar lymphadenopathy and a characteristic "targetoid" appearance on chest X-ray, while HP is characterized by a more patchy distribution and often presents in patients with a history of occupational exposure to antigens.

**Why Each Wrong Option is Incorrect:**

A. Allergic bronchopulmonary aspergillosis (ABPA) is a severe allergic reaction to the fungus Aspergillus fumigatus, usually seen in patients with asthma or cystic fibrosis. It does not typically present with lung fibrosis, but rather with bronchial wall thickening and airway obstruction.

B. Drug-induced lung disease (DILD) is a group of interstitial lung diseases caused by medications or toxins. Unlike HP, DILD usually presents with a more acute onset of symptoms and is associated with the use of a specific drug or toxin.

D. Pneumoconiosis is a group of lung diseases caused by inhalation of dusts, fibers, or other particulate matter. Unlike HP, pneumoconiosis is associated with occupational exposures and presents with diffuse parenchymal opacities on chest X-ray.

**Clinical Pearl:** HP is a distinct entity with specific clinical, radiological, and pathological features that differentiate it from other interstitial lung diseases. A detailed history, including occupational exposure, is crucial in diagnosing HP.

**Correct Answer:** C. Sarcoidosis is a common granulomatous interstitial lung disease of unknown origin. It presents with bilateral hilar lymphadenopathy and characteristic "targetoid" appearance on chest X-ray. The key is the presence of granulomas and the clinical context of the patient's occupation, such as exposure to dust or other antigens.