A 52-year-old woman with stage 3 chronic kidney disease and hypertension presents with bilateral ankle edema and dyspnea. Chest X-ray shows bilateral pleural effusions with preserved costophrenic angles and normal cardiac silhouette. Thoracentesis is performed on the right side. The pleural fluid analysis shows: protein 2.8 g/dL, LDH 140 IU/L, serum protein 6.2 g/dL, serum LDH 180 IU/L. Which of the following imaging findings would MOST reliably differentiate this transudative effusion from an exudative effusion in this patient?