## Core Concept
The question assesses understanding of lung function tests in obstructive lung disease. Obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD) and asthma, are characterized by airflow limitation. This limitation is not fully reversible and is usually progressive. It results from a combination of airway and parenchymal damage.

## Why the Correct Answer is Right
In moderate severity obstructive lung disease, patients typically exhibit a decrease in Forced Expiratory Volume in 1 second (FEV1) and a decrease in the ratio of FEV1 to Forced Vital Capacity (FVC), which is known as the FEV1/FVC ratio. The FEV1/FVC ratio is decreased because FEV1 decreases more significantly than FVC. However, the **Total Lung Capacity (TLC)** may be normal or increased due to air trapping. The **Residual Volume (RV)** and **RV/TLC ratio** are typically increased, indicating air trapping and hyperinflation of the lungs.

## Why Each Wrong Option is Incorrect
- **Option A:** Decrease in FEV1 - This is a characteristic feature of obstructive lung disease, reflecting the obstruction to airflow.
- **Option B:** Decrease in FEV1/FVC ratio - This is also typical, indicating that the obstruction affects the ability to exhale quickly.
- **Option D:** Increase in RV/TLC ratio - This is expected due to air trapping.

## Why Option C is Incorrect (The Correct Answer)
- **Option C:** Decrease in TLC - This option is incorrect in the context of moderate obstructive lung disease. While mild obstructive lung disease might have a normal TLC, moderate to severe disease often shows a normal or increased TLC due to hyperinflation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that in obstructive lung diseases, the lung volumes (like TLC) can be normal or increased, whereas in restrictive lung diseases, TLC is decreased. This distinction is critical for differentiating between obstructive and restrictive lung diseases based on pulmonary function tests.

## Correct Answer: C. Decrease in TLC.