This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
[{'content': 'A 70-year-old nursing home patient refused the influenza vaccine and subsequently developed influenza. She died of acute pneumonia 1 week after contracting the "flu." The most common cause of acute post-influenzal pneumonia is \nSelect and explain. \nA. Legionella, B. Listeria, C. Staphylococcus aureus, D. Klebsiella', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Staphylococcus aureus is the most common cause of post-influenzal secondary bacterial pneumonia. It most often affects the elderly, although patients of any age may be afflicted. The pneumococcus as well as group A streptococci and Haemophilus influenzae may also cause pneumonia.', 'role': 'assistant'}]