Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The visualized mediastinal structures are unremarkable . There is increased opacification of the retrocardiac area when compared with prior film . Additionally , there is poor delineation of the left hemidiaphragm . Given leukocytosis , these findings are concerning for left - sided pneumonia .
    OUTPUT:
    
Retrocardiac opacity concerning for left lower lung pneumonia .