Pleural Effusion — Imaging

The lateral chest X-ray shown above demonstrates blunting of the posterior costophrenic angle, with fluid accumulation indicated by arrows A and B. What is the most likely diagnosis?

Which radiographic sign indicates that a pleural effusion is free-flowing rather than loculated on a frontal chest X-ray?

A 52-year-old man with a 10-year history of rheumatoid arthritis presents with progressive dyspnea and pleuritic chest pain for 3 weeks. Examination reveals reduced breath sounds at the right base. Chest X-ray shows a moderate right-sided pleural effusion with an underlying opacity in the right lower lobe. Thoracentesis yields exudative fluid with low glucose (18 mg/dL), elevated LDH (850 IU/L), and low complement levels. Which imaging feature on CT chest would be most specific for the underlying diagnosis?

A 68-year-old woman with a 6-month history of progressive dyspnea and weight loss presents with a unilateral right-sided pleural effusion on chest X-ray. Thoracentesis shows an exudative fluid with positive cytology for adenocarcinoma. CT chest reveals a 3 cm mass in the right lower lobe with irregular borders, mediastinal lymphadenopathy, and a moderate right pleural effusion. The pleural fluid also shows elevated LDH (950 IU/L) and low pH (6.8). What is the most important imaging finding that would change the staging and treatment approach?

A 52-year-old man with a 3-week history of progressive dyspnea and chest pain presents to the emergency department. Chest X-ray shows a moderate right-sided pleural effusion with blunting of the costophrenic angle. Clinical examination reveals dullness to percussion and decreased breath sounds on the right side. The patient is hemodynamically stable. Which investigation is most appropriate for determining the nature of this pleural effusion?