Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: PA and lateral views of the chest are provided . Opacity at the left lung base could represent atelectasis versus pneumonia . There is likely a small left pleural effusion . The right lung appears clear , though volumes are low . The cardiomediastinal silhouette is stable . A retrocardiac density is noted which could reflect the presence of a hiatal hernia . Bony structures appear intact with degenerative changes in the lower thoracic spine and upper lumbar spine .
    OUTPUT:
    
Left basal opacity , likely atelectasis , less likely pneumonia .