The description of blunting of the costophrenic angle (which requires approximately 75-100 mL of fluid to be visible on an upright PA view) and a curvilinear opacity (often referred to as the 'meniscus sign') extending upwards along the lateral chest wall is pathognomonic for pleural effusion on a chest X-ray. Pneumothorax would show a visceral pleural line and absence of lung markings. Consolidation would be a parenchymal opacity within the lung parenchyma, and atelectasis would show volume loss, often with compensatory hyperinflation of adjacent lung.
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