Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Frontal and lateral chest radiographs demonstrate a normal cardiomediastinal silhouette and fairly well - aerated lungs . There is increased opacity projecting over the lateral left lung base . There is no appreciable pleural effusion or pneumothorax . The visualized upper abdomen is unremarkable .
    OUTPUT:
    
Increased opacity projecting over the lateral left lung base may reflect atelectasis , but an infectious process can not be excluded . Particularly given history of no improvement on antibiotics , postobstructive pneumonia is a concern .