A 68-year-old woman with no significant past medical history presents with progressive dyspnea and right-sided pleuritic chest pain for 6 weeks. Chest X-ray reveals a large right pleural effusion. Thoracentesis yields straw-colored fluid. Pleural fluid analysis shows: protein 3.8 g/dL, LDH 210 IU/L, cell count 150 WBC/μL (80% lymphocytes), ADA 12 U/L, negative Gram stain and culture. Serum protein is 7.0 g/dL, serum LDH is 380 IU/L. CT chest shows no lung mass. What is the most appropriate next step in management?