Presentation
Respiratory distress with cough for several weeks, but marked clinical deterioration over 24 hours.
Patient Data

Complete opacification of the left hemithorax with contralateral mediastinal shift consistent with a large pleural effusion exerting positive mass effect.

The left hemithorax is much more dense than it should be and there is virtually no aerated lung visible.
The midline (dashed line) is identified by looking for the spinous processes. The trachea (shaded) is clearly pushed to the right (away from the dense hemithorax).
Case Discussion
This large left-sided complete white-out demonstrates positive mass effect causing mediastinal shift.
There are several causes of a hemithorax white-out including lung collapse, consolidation and pleural effusion. Because of the mass effect demonstrated, a large pleural effusion is the most likely cause.
The causes of pleural effusions can be separated into those that cause a transudate and those that cause an exudate. In children, the most likely cause by far is a parapneumonic effusion.