This patient is a girl who had suffered penetrating injury to the chest by a tree branch while horseback riding.
The radiograph is obtained as a portable supine study in the intensive care unit. There is a complete lack of lung markings on the left side, due to filling of the pleural space with fluid (blood) and air (pneumothorax). The resulting density in the pleural space is therefore intermediate between water density (fluid or blood) and air density (pneumothorax). The left lung is collapsed, therefore the accumulation of fluid and air in the left pleural space does not result in mediastinal shift. The homogeneous opacity in the left hemithorax, devoid of lung markings, is the finding that leads to the correct diagnosis. In the setting of penetrating trauma, the fluid is assumed to be blood rather than pleural effusion.