Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is a background of chronically increased interstitial markings . There is increased heterogeneous density in the left lower lobe which is likely secondary to atelectasis . The lungs are otherwise clear . There is no pneumothorax pleural effusion . Heart size is normal . Pulmonary vascularity is normal . Degenerative changes of the right acromioclavicular joint are noted .
    OUTPUT:
    
No definitive evidence of pneumonia . Left lower lobe opacity likely secondary to atelectasis but early pneumonia can not be excluded .