Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Left upper lobe opacity suggests pneumonia in the given clinical history . Retrocardiac opacity is most likely atelectasis from lack of full inspiration . No pulmonary edema , pleural effusion or pneumothorax . The heart size , hila , and pleura are normal .
    OUTPUT:
    
Left upper lobe pneumonia . Follow - up chest x-ray after treatment to ensure resolution .