Which of the following is a sign of a tension pneumothorax?

A tension pneumothorax occurs when air enters the pleural space and cannot escape, leading to increased pressure that collapses the lung on the affected side and can compress the mediastinum. One of the key signs of this condition is the unequal rise and fall of the chest. This occurs because the affected lung cannot expand due to the pressure buildup in the pleural cavity, causing the chest to appear asymmetrical.

When the chest rises and falls unevenly, it directly indicates that one side is compromised, which is typical in tension pneumothorax cases. Monitoring this sign is crucial for early detection and prompt treatment, as it is a life-threatening emergency.

In contrast, the other options are associated with different conditions or responses but don't specifically indicate a tension pneumothorax. For instance, a weak or thready pulse might suggest shock or reduced cardiac output, bilateral breath sounds can be heard in several respiratory conditions, and hyperventilation may occur due to anxiety or other issues but isn't a definitive sign of tension pneumothorax. Therefore, recognizing the unequal rise and fall of the chest is vital for identifying this critical medical emergency.