Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: A left retrocardiac opacity likely represents a small hiatal hernia with adjacent compression atelectasis . Linear atelectasis is noted at the peripheral left lung base . The upper lungs are grossly clear . There is no appreciable pleural effusion , pneumothorax , or pulmonary edema . Calcifications are noted at the aortic arch . The cardiomediastinal silhouette is unremarkable in appearance . There is no evidence of free subdiaphragmatic air .
    OUTPUT:
    
Left retrocardiac opacity likely represents a small hiatal hernia with adjacent atelectasis . However , superimposed infection is difficult to exclude completely .