Clearly there is increased attenuation on the left. But is there consolidation, effusion or collapse?
All three of the above can cause generalised opacification and to narrow the differential we need to look at the rest of the film. There is volume loss on the left: mediastinum is shifted to the left (look at the right heart border) and rib spaces are narrowed. An effusion would cause increased volume and consolidation preserves volume. So, the underlying problem here is collapse.
What has caused the collapse?
In the midline, an endotracheal tube can be seen projected over the upper mediastinum. However, it continues down the right main bronchus and therefore, only the right lung is being aerated.
Supine chest x-ray in an intubated patient, demonstrates the endotracheal tube located in the right main bronchus, probably past the origin of the right upper lobe bronchus. There is associated complete atelectasis of the left lung with marked shift of the mediastinum towards the left. Nasogastric tube in situ.