Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart size is top normal . Cardiomediastinal contours are stable . Opacity at the left lung base is likely secondary to atelectasis . There is no pleural effusion or pneumothorax . The visualized osseous structures are unremarkable .
    OUTPUT:
    
Opacity at the left lung base is likely secondary to atelectasis .