Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Subtle left base opacity could be due to atelectasis , but pneumonia is not excluded in the appropriate clinical setting . No pleural effusion or pneumothorax is seen . The cardiac and mediastinal silhouettes are stable .
    OUTPUT:
    
Subtle left base opacity could be due to atelectasis , but pneumonia is not excluded in the appropriate clinical setting .