**Core Concept**
The FEV1/FVC ratio is a vital parameter in pulmonary function tests (PFTs) that assesses the efficiency of airway emptying. It is the ratio of the forced expiratory volume in one second (FEV1) to the forced vital capacity (FVC). A decrease in this ratio indicates an obstructive lung disease.

**Why the Correct Answer is Right**
The FEV1/FVC ratio is decreased in obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD) and asthma. This is because these conditions cause airway narrowing and inflammation, leading to a slower air flow rate. As a result, the amount of air that can be forcibly exhaled in one second (FEV1) is decreased, while the total lung capacity (FVC) remains relatively normal. This leads to a decreased FEV1/FVC ratio.

**Why Each Wrong Option is Incorrect**
**Option A:** Emphysema, a type of COPD, actually increases the FEV1/FVC ratio due to air trapping and overinflation of the lungs.
**Option B:** Pulmonary fibrosis, a type of restrictive lung disease, would decrease the FVC more significantly than the FEV1, resulting in an increased FEV1/FVC ratio.
**Option C:** Pneumonia would initially decrease the FEV1 due to inflammation and consolidation of lung tissue, but it would also decrease the FVC, resulting in a relatively normal FEV1/FVC ratio.

**Clinical Pearl / High-Yield Fact**
Remember the "Obstruction" mnemonic: Obstruction (COPD, asthma) decreases FEV1/FVC; Restriction (pulmonary fibrosis) increases FEV1/FVC.

**Correct Answer:** C.