**Core Concept**
A pneumothorax occurs when air enters the pleural space, causing the lung to collapse partially or completely. This can lead to a decrease in lung volume, displacement of the mediastinum, and potentially life-threatening complications if left untreated.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of pneumothorax. In a tension pneumothorax, air enters the pleural space but cannot escape, leading to increased intrathoracic pressure on the affected side. This causes the mediastinum to shift away from the affected side, compromising cardiac output and venous return. The increased pressure also causes the lung to collapse further, leading to hypoxia.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because a spontaneous pneumothorax typically occurs in a person with no underlying lung disease, whereas a traumatic pneumothorax is caused by an external injury.

**Option B:** This statement is incorrect because a pneumothorax is characterized by air in the pleural space, not fluid.

**Option C:** This statement is incorrect because a hemothorax is characterized by blood in the pleural space, not air.

**Clinical Pearl / High-Yield Fact**
A pneumothorax can be a life-threatening emergency, and prompt recognition and treatment are essential to prevent complications. The classic teaching is to treat a tension pneumothorax with needle decompression, followed by chest tube insertion.

**Correct Answer: D. A tension pneumothorax is characterized by increased intrathoracic pressure on the affected side, leading to mediastinal shift and compromised cardiac output.**