A 68-year-old woman with no significant past medical history presents with a 3-week history of progressive dyspnea. Chest X-ray shows a large right-sided pleural effusion. Thoracentesis yields clear, straw-colored fluid. Pleural fluid analysis: protein 2.1 g/dL (serum 7.0 g/dL), LDH 150 IU/L (serum 300 IU/L), glucose 95 mg/dL, pH 7.4, ADA <5 U/L, negative for malignant cells on cytology. What is the most appropriate next step in management?