What condition is characterized by a collection of pus in the pleural cavity?

Empyema is the correct answer, as it specifically refers to the accumulation of pus in the pleural cavity, the space between the lungs and the chest wall. This condition often occurs as a complication of pneumonia, lung abscesses, or chest surgery, leading to infection and inflammation in the pleural space. The presence of pus indicates that the body is responding to an infection, and as a result, it can lead to significant respiratory distress if not treated effectively.

In contrast, pneumothorax involves air entering the pleural space, which can cause lung collapse but does not involve pus. Acute bronchitis is an inflammation of the bronchial tubes, often due to infection, but it does not pertain to the pleural cavity or the accumulation of pus. Chronic obstructive pulmonary disease (COPD) encompasses a group of lung conditions that cause breathing difficulties over time but is not characterized by pus accumulation in the pleural space. Thus, empyema is uniquely defined by the presence of pus in the pleural cavity.