Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Single AP portable upright chest radiograph provided . Lung volumes are low with bronchovascular crowding at the lung bases . No effusion or pneumothorax is seen . No free air below the right hemidiaphragm . Cardiomediastinal silhouette appears stable . Bony structures are intact .
    OUTPUT:
    
Left basal opacity likely represents atelectasis versus bronchovascular crowding , less likely pneumonia .