**Core Concept**
A pneumothorax is a condition where air enters the pleural space, leading to increased pressure and potential collapse of the lung. This patient's presentation is consistent with a traumatic pneumothorax, which requires prompt intervention to prevent further complications.

**Why the Correct Answer is Right**
The expected complication of a traumatic pneumothorax is a condition known as **tension pneumothorax**. This occurs when air continues to enter the pleural space but is unable to escape, leading to a gradual increase in pressure. As pressure builds, the mediastinum is shifted away from the affected side, compromising the patient's cardiovascular and respiratory systems.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Hemothorax** is a condition where blood enters the pleural space, often due to trauma. While it is a possible complication of chest trauma, it is not the expected complication of pneumothorax.
* **Option B:** **Pulmonary contusion** is a bruise of the lung tissue, often caused by blunt trauma. It can occur in conjunction with pneumothorax but is not a direct complication of air entering the pleural space.
* **Option D:** **Atelectasis** is a condition where a portion of the lung collapses or is not inflated. While it can occur in patients with pneumothorax, it is not the expected complication in this scenario.

**Clinical Pearl / High-Yield Fact**
In patients with a pneumothorax, it is essential to monitor for signs of tension pneumothorax, including increased respiratory distress, decreased blood pressure, and tracheal deviation away from the affected side.

**Correct Answer:** C.