The chest x-ray shows a double left heart border.
The double left heart border (sail sign) along with silhouetting of the left medial hemi -diaphragm and depressed left main bronchus is suggestive of left lower lobe collapse.
The left lower lobe collapse in the above patient was due to mucus plugging. Other causes are endobronchial lesions occluding the bronchus, parenchymal mass causing external pressure on the bronchus and inhaled foreign body.
Reference: Fundamentals of Diagnostic Radiology by Brant & Helms, 4th Edition.