The frontal chest x-ray shows ‘sail sign’ related to left lower lobe collapse. There is a triangular density behind the heart, causing a double left heart border. The left main bronchus is depressed.

Causes of left lower lobe collapse are –

• Endobronchial lesion.
• Mucus plug.
• Foreign body in the left main bronchus.

Metastatic deposits were the reason for left lower lobe collapse in the above patient.

Reference: Grainger & Allison’s Diagnostic Radiology.