Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: PA and lateral views of the chest are compared to previous exam from ___ . Subtle opacity is noted at the left lung base which may represent atelectasis versus early pneumonia . Posterior costophrenic angles are sharp . Elsewhere , the lungs are clear noting minimal biapical scarring . The cardiomediastinal silhouette is within normal limits . Osseous and soft tissue structures are unremarkable .
    OUTPUT:
    
Left base atelectasis or early pneumonia .