**Core Concept**
Decreased lung compliance refers to the reduced ability of the lungs to expand and fill with air, resulting in an increased effort required to breathe. This condition is often associated with conditions that affect the lung parenchyma, pleura, or chest wall, leading to a stiffened lung that is difficult to inflate.

**Why the Correct Answer is Right**
In decreased lung compliance, the lung's ability to expand is impaired, causing an increase in the pressure required to inflate the lungs. This can be due to various factors, such as pulmonary fibrosis, pleural effusion, or chest wall deformities. The increased pressure required to inflate the lungs can lead to respiratory distress and fatigue. The lung's compliance is determined by the elastic properties of the lung tissue, the surface tension of the alveolar fluid, and the pressure required to overcome the resistance to expansion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased lung compliance is the opposite of decreased lung compliance, where the lungs are able to expand more easily, reducing the effort required to breathe.
* **Option B:** Pneumothorax is a condition where air enters the pleural space, leading to lung collapse, but it is not directly related to decreased lung compliance.
* **Option C:** Obstructive lung disease, such as chronic obstructive pulmonary disease (COPD), is characterized by airway obstruction, which can lead to increased airway resistance, but it is not the same as decreased lung compliance.

**Clinical Pearl / High-Yield Fact**
Decreased lung compliance can be caused by various conditions, but it is often associated with conditions that affect the lung parenchyma or pleura, such as pulmonary fibrosis, pleural effusion, or pneumothorax.

**Correct Answer: D. Decreased lung compliance leads to an increased effort required to breathe due to the stiffened lung, resulting in respiratory distress and fatigue.**