Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is a retrocardiac opacity obscuring the left hemidiaphragmatic contour . This may reflect pleural effusion with atelectasis or pneumonia . Mild cardiomegaly is stable . There is no pneumothorax . Pulmonary vascularity is normal .
    OUTPUT:
    
Retrocardiac opacity likely represents pleural effusion and atelectasis but left lower lobe pneumonia in the appropriate clinical setting can not be excluded .