The chest x-ray shows a plate-like atelectasis in the left lower zone.

Plate-like atelectasis is a common finding on the chest x-ray. Seen as horizontal linear area of increased density at the lung bases.

Plate-like atelectasis in most cases has no clinical significance. It can be seen in basal hypoventilation (post operative state, obesity, acute abdomen). It can also be a chest x-ray finding in pulmonary embolism (Fleischner’s line).

The above patient had a segmental pulmonary embolism diagnosed on CT pulmonary angiogram.

Reference: http://www.radiologyassistant.nl/en/p50d95b0ab4b90/chest-x-ray-lung-disease.html