**Core Concept**
Hypersensitivity pneumonitis (HP) is a lung disease characterized by an immune-mediated response to inhaled antigens, leading to inflammation and fibrosis in the lung parenchyma. The diagnosis of HP often relies on a combination of clinical, radiological, and pathological findings.

**Why the Correct Answer is Right**
The best diagnostic test for HP is the **high-resolution computed tomography (HRCT) scan**. HRCT scans can show characteristic lung patterns, including ground-glass opacification, reticular patterns, and bronchiectasis, which are indicative of HP. The HRCT scan is particularly useful in differentiating HP from other interstitial lung diseases. Additionally, the presence of characteristic lung patterns on HRCT scan can guide further diagnostic testing, such as bronchoalveolar lavage (BAL) or surgical lung biopsy.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests, such as autoimmune antibody profiles, are not specific for HP and can be elevated in other conditions as well.
**Option B:** Chest X-ray may show nonspecific findings, such as reticulonodular patterns, but is not sensitive or specific enough for HP diagnosis.
**Option C:** Pulmonary function tests (PFTs) can show restrictive lung disease, but are not diagnostic for HP and can be seen in other conditions, such as idiopathic pulmonary fibrosis.

**Clinical Pearl / High-Yield Fact**
HRCT scan is the imaging modality of choice for diagnosing HP, as it can show characteristic lung patterns that are not visible on chest X-ray or other imaging modalities.

**Correct Answer:** C.