What is the effect of pneumothorax noted in a chest radiograph?

A pneumothorax occurs when air escapes into the pleural space, which is the area between the lung and the chest wall. This accumulation of air can lead to the collapse of the lung on the affected side. In a chest radiograph, the findings typically include a visible line that indicates the pleura's edge, with the affected lung appearing smaller and more retracted. This is characterized by decreased lung volume and loss of vascular markings due to the lung being unable to fully expand.

While lung hyperinflation and normal lung inflation reflect healthy lung function, they do not accurately depict a pneumothorax. Fluid accumulation is associated with conditions like pleural effusion and does not represent the air presence characteristic of a pneumothorax. Thus, the identification of a collapsed lung on a radiograph is a direct consequence of the air entering the pleural space, making it the most appropriate answer.