Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Since the chest radiographs obtained 3 days prior , there has been a significant increase in left lung atelectasis with leftward mediastinal shift . Patient positioning does not account for all apparent mediastinal shift . Unable to assess for concomitant left pleural effusions or consolidation . The right lung is fully expanded and clear .
    OUTPUT:
    
Subtotal left lung collapse with significant leftward mediastinal shift concerning for an airway obstruction such as an endobronchial lesion , foreign body , or mucous plug .