A 58-year-old man with known cirrhosis presents with dyspnea and right-sided pleuritic chest pain. Chest X-ray shows a large right pleural effusion with blunting of the costophrenic angle. Ultrasound of the abdomen confirms ascites. Diagnostic thoracentesis is performed. The pleural fluid analysis shows: protein 2.8 g/dL, LDH 180 IU/L, serum protein 6.2 g/dL, serum LDH 420 IU/L. Which of the following radiological signs would be MOST helpful in differentiating this effusion from a parapneumonic effusion in the same patient who develops pneumonia?