**Core Concept**
The most common cause of death in blunt trauma to the chest is due to severe disruption of the mediastinal structures, leading to cardiovascular collapse. The mediastinum contains vital structures such as the heart, major blood vessels, and trachea.

**Why the Correct Answer is Right**
The primary mechanism of injury in blunt chest trauma is the transmission of force to the mediastinum, resulting in shearing forces that disrupt the vascular and cardiac structures. This disruption can lead to severe bleeding, cardiac tamponade, and airway obstruction. The severity of the injury depends on the magnitude and direction of the force applied. The heart and major blood vessels are particularly vulnerable to injury due to their location in the mediastinum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while pulmonary contusion can be a significant cause of morbidity in blunt chest trauma, it is not the most common cause of death.
**Option B:** This option is incorrect because while aortic injury is a serious complication of blunt chest trauma, it is not the most common cause of immediate death.
**Option C:** This option is incorrect because while cardiac arrest can occur due to blunt chest trauma, it is not the primary cause of death.

**Clinical Pearl / High-Yield Fact**
The "seatbelt sign" is a classic sign of blunt chest trauma, indicating injury to the anterior chest wall. This sign is associated with severe mediastinal injury and increased risk of mortality.

**Correct Answer: C. Cardiac tamponade.**