What causes pleural effusion?

Pleural effusion refers specifically to the accumulation of fluid in the pleural space, the area between the layers of the pleura that surround the lungs. This condition can result from a variety of underlying causes, including infections, heart failure, malignancies, or inflammatory diseases. The key factor in pleural effusion is the excess fluid in the pleural cavity, which can lead to symptoms such as difficulty breathing, chest pain, and a dry cough.

While infections of the lungs, such as pneumonia, can lead to pleural effusion either directly or indirectly, they themselves are not the cause of pleural effusion. Similarly, a respiratory infection may lead to fluid accumulation, but the term itself captures a broader category of conditions. Air trapped in the thoracic cavity describes a different condition known as pneumothorax, which involves the presence of air rather than fluid. Thus, the defining feature of pleural effusion is the presence of fluid accumulation, making that the correct answer.