Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The cardiomediastinal and hilar contours are within normal limits . There is calcification of the aortic knob . The lungs are well expanded . There is an increased opacity at the left lung base . The right lung is clear . There is no pleural effusion or pneumothorax . There is no overt pulmonary edema .
    OUTPUT:
    
Left lung base opacity could reflect atelectasis . However , in the appropriate clinical setting , an infectious process can not be excluded .