Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Subtle left base retrocardiac opacity is seen which while could be due to atelectasis , underlying pneumonia is not excluded in the appropriate clinical setting . The right lung is clear . There is no pleural effusion or pneumothorax . The cardiac and mediastinal silhouettes are unremarkable . No overt pulmonary edema is seen .
    OUTPUT:
    
Subtle left base retrocardiac opacity , while could be due to atelectasis , infection or aspiration not excluded in the appropriate clinical setting .