What are some of the causes of a unilateral white-out?
Total lung collapse, massive air-space opacification (pneumonia), and pleural effusion.
What makes pleural effusion the likely cause here?
The degree of mass effect produced by the left-sided pathology makes pleural effusion most likely. Collapse would cause volume loss and airspace filling is usually mass-neutral.
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).