Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Left lower lobe opacity is improving peripherally , but appears more confluent in the central left retrocardiac region . Lungs are otherwise clear , and there are no definite pleural effusion . Cardiomediastinal contours are normal .
    OUTPUT:
    
Worsening left retrocardiac opacity , which may be due to atelectasis , aspiration , or developing infection .