**Core Concept**
Obstructive pulmonary disease (OPD) refers to a group of conditions characterized by the narrowing or blockage of the airways, leading to impaired airflow and breathing difficulties. The hallmark of OPD is an obstructive pattern on pulmonary function tests (PFTs), which indicates a problem with airway patency rather than lung parenchyma.

**Why the Correct Answer is Right**
Reduced residual volume (RV) is not a characteristic of obstructive pulmonary disease. In fact, RV is often increased in OPD due to air trapping and hyperinflation of the lungs. FEV1 (forced expiratory volume in 1 second) is reduced in OPD, reflecting the narrowing of airways. Diffusion capacity (DLCO) is also reduced in OPD due to the loss of lung surface area and the presence of inflammation. Mid-expiratory flow rate (MEFR) is typically reduced in OPD, indicating impaired airflow.

**Why Each Wrong Option is Incorrect**
**Option A:** Reduced FEV1 is a characteristic of obstructive pulmonary disease, reflecting the narrowing of airways.
**Option B:** Reduced diffusion capacity is a characteristic of obstructive pulmonary disease, indicating impaired gas exchange.
**Option D:** Reduced mid-expiratory flow rate is a characteristic of obstructive pulmonary disease, reflecting impaired airflow.

**Clinical Pearl / High-Yield Fact**
In OPD, the ratio of FEV1 to forced vital capacity (FVC) is typically reduced, indicating an obstructive pattern. This ratio is a key diagnostic criterion for OPD and is used to distinguish it from restrictive lung diseases.

✓ Correct Answer: C. Reduced residual volume