Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Cardiomediastinal contours are stable in appearance . A confluent opacity in the left retrocardiac region has slightly improved , but is difficult to fully characterize on this single projection . The right lung remains slightly overinflated but grossly clear .
    OUTPUT:
    
Improving left lower lobe opacity . Considering rapid development and rapid improvement , aspiration is a likely possible cause . However , followup radiographs with PA and lateral technique would be helpful as well as clinical correlation to exclude an infectious pneumonia .