**Core Concept**
Interstitial lung disease (ILD) is a broad category of lung disorders characterized by inflammation and scarring of the lung tissue. The clinical presentation of ILD can vary widely, but it often includes progressive dyspnea, cough, and chest X-ray (CXR) findings of interstitial opacities. Spirometry in ILD can show a restrictive pattern, but in some cases, an obstructive pattern may be seen due to the involvement of smaller airways.

**Why the Correct Answer is Right**
The patient's presentation of progressive dyspnea and CXR findings of interstitial opacities are suggestive of ILD. The obstructive pattern on spirometry may be due to the involvement of smaller airways in some ILD conditions, such as hypersensitivity pneumonitis or cryptogenic organizing pneumonia. These conditions can cause inflammation and scarring of the airways, leading to an obstructive pattern on spirometry.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pulmonary embolism typically presents with acute onset of dyspnea, pleuritic chest pain, and tachycardia, rather than progressive dyspnea over one year.
* **Option B:** This option is incorrect because asthma typically presents with episodic wheezing, cough, and dyspnea, rather than progressive dyspnea over one year. Spirometry in asthma shows an obstructive pattern, but the pattern is typically more reversible than in this patient.
* **Option C:** This option is incorrect because chronic obstructive pulmonary disease (COPD) typically presents with a history of smoking and a history of progressive dyspnea over many years. However, the spirometry pattern in COPD is typically obstructive, but with a more pronounced decrease in FEV1/FVC ratio.

**Clinical Pearl / High-Yield Fact**
ILD can be a challenging diagnosis to make, and a high index of suspicion is required. A thorough history and physical examination, combined with imaging studies and spirometry, are essential in making the diagnosis.

**Correct Answer:** C.