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
Empyema thoracis is most commonly caused by which organism - 
Select and explain. 
A. Streptococcus pneumoniae, B. Pseudomonas, C. Mycoplasma, D. Stapylococcus aureus
A: Streptococcus pneumoniae
Answer: A: Streptococcus pneumoniae 
Explanation: None