Lung infiltrates are best described as which of the following?

• A. A substance denser than air, such as pus, blood, or protein, which lingers within the parenchyma of the lungs
• B. Air in the pleural space
• C. Fluid in the pericardial sac
• D. Air in the alveolar spaces

Answer: (D)

Infiltrates describe areas in the lung where material denser than air occupies the lung parenchyma, causing increased density on imaging. This density comes from substances such as pus, blood, or protein that fill the alveoli or seep into the interstitium, replacing the normal air and producing an opacity. Think of it as anything inside the lung tissue that makes it appear whiter on a chest radiograph, as seen with pneumonia (alveolar exudate), pulmonary edema, or hemorrhage.

By contrast, air in the pleural space is a pneumothorax, fluid in the pericardial sac is a pericardial effusion, and air in the alveolar spaces describes normal air-filled alveoli or non-infiltrative conditions. Thus, infiltrates are best described as substances denser than air occupying the lung tissue.