Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Heart size is upper limits of normal . The mediastinal and hilar contours are normal . The pulmonary vasculature is normal . Lungs are remarkable for a subtle area of increased opacity in left retrocardiac region , projecting over the lower thoracic spine on the lateral view . . No pleural effusion or pneumothorax is seen . There are no acute osseous abnormalities .
    OUTPUT:
    
Left lower lobe posterior basilar opacity , concerning for pneumonia in the setting of a COPD exacerbation .