Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The left lung base is densely opacified , possibly a combination of atelectasis , consolidation , or pleural fluid . Right lung is fully expanded and clear . No pneumothorax . Moderate cardiomegaly with mild pulmonary vascular congestion .
    OUTPUT:
    
Probable moderate left pleural effusion with left lower lobe atelectasis or , alternatively , developing pneumonia .