Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lung volumes are low . In the left lower lung zone , there is an ill - defined retrocardiac opacifity which is likely atelectasis , but early or developing pneumonia can not be excluded . There is no pulmonary edema , pleural effusion , or pneumothorax . The cardiomediastinal silhouette is normal .
    OUTPUT:
    
Ill defined left lower lung zone opacity is likely atelectasis , although early or developing pneumonia can not be excluded .