Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is subtle left lower lung opacity , which could represent atelectasis or pneumonia . No pleural effusion , pneumothorax , or pulmonary edema is detected . Heart and mediastinal contours are stable with mild cardiomegaly and aortic tortuosity .
    OUTPUT:
    
Subtle left lower lung opacity , which could represent atelectasis or pneumonia .