This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'Empyema thoracis is most commonly caused by which organism - \nSelect and explain. \nA. Streptococcus pneumoniae, B. Pseudomonas, C. Mycoplasma, D. Stapylococcus aureus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]