A frontal chest radiograph shows a well-defined triangular opacity projecting below and medial to the left cardiac border, obscuring the left hemidiaphragm, with loss of the retrocardiac vascular markings. The most likely diagnosis is:

• A Left pleural effusion
• B Left upper lobe consolidation
• C Pericardial effusion
• D Left lower lobe collapse ✓

Explanation

Left lower lobe collapse produces the sail sign, a triangular opacity behind the heart that projects inferomedially, blurs the medial hemidiaphragm and increases retrocardiac density. On the lateral film there is a posteriorly placed triangular opacity with inferior displacement of the major fissure. Left upper lobe disease would obscure the left heart border itself, effusion blunts the costophrenic angle first, and pericardial effusion gives a globular cardiomegaly without a triangular retrocardiac shadow.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.