Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart size is normal . The hilar and mediastinal contours are normal . The base of the left lung demonstrates irregular opacities . There is no pleural effusion or pneumothorax . The visualized osseous structures are unremarkable .
    OUTPUT:
    
Opacity at the left lung base is of unclear etiology . Could represent atelectasis , peripheral lung infarction from an pulmonary embolus . An acute infectious process such as a viral etiology can not be excluded .