Which condition occurs when infection leads to pus in the pleural cavity?

Empyema is the correct choice as it specifically describes a condition where there is an accumulation of pus in the pleural cavity, usually as a result of an infection. This condition occurs when bacteria or other pathogens infect the pleural space, leading to an inflammatory response and the formation of pus.

The presence of pus in this area can lead to severe complications, including respiratory distress and the inability of the lungs to expand properly. Treatment often involves drainage of the pus and the use of antibiotics to manage the underlying infection.

In contrast, the other conditions listed—pertussis, bronchitis, and tonsillitis—relate to infections of the respiratory system but do not involve the pleural cavity or result in empyema. Pertussis is whooping cough caused by a specific bacteria, bronchitis involves inflammation of the bronchi, and tonsillitis refers to the inflammation of the tonsils. None of these conditions result in the accumulation of pus in the pleural cavity, distinguishing empyema as the correct answer.