Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lung volumes are normal . There is a retrocardiac opacity best seen on frontal views as slightly obscures the medial last hemidiaphragm which may represent atelectasis however more likely pneumonia in the appropriate clinical setting . The cardiomediastinal hilar contours are normal . The pleural surfaces are normal .
    OUTPUT:
    
Retrocardiac opacity concerning for a left lower lobe pneumonia .