Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The cardiac , mediastinal and hilar contours appear stable . There is an opacity in the posterior lower medial left lower lobe silhouetting the hemidiaphragm and also visible as a retrocardiac opacity on the frontal view , most suggestive of pneumonia . Elsewhere , the lungs appear clear . It is difficult to exclude the coinciding presence of a very small pleural effusion on the right , although a substantial one is doubted . The posterior right costophrenic sulcus is sharp .
    OUTPUT:
    
Opacity in the left lower lobe concerning for pneumonia . Substantial pleural effusion is doubtful .