A pleural effusion is best demonstrated by

• A. Two right-angle radiographs
• B. PA and AP films
• C. A lateral decubitus film
• D. A lateral neck film

Answer: (C)

The key idea is that pleural fluid settles in the dependent pleural space and is most easily seen when gravity is allowed to act on it. A lateral decubitus radiograph places the patient on their side, so the fluid layers between the lung and the chest wall and forms a horizontal fluid level along the dependent side. This layering makes even small volumes of free fluid visible as a distinct meniscus or fluid line that is not as readily seen on standard upright PA/AP views, where the posterior sulci can be obscured by the heart and mediastinal structures.

The other approaches are less sensitive for small effusions: upright chest views can miss small amounts, and a lateral neck film does not evaluate the chest for pleural fluid. The lateral decubitus view specifically enhances detection by exploiting gravity to reveal the fluid layer.