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.