Septic arthritis

A 32-year old man comes to the emergency department because of fever,  worsening pain, and impaired range of motion in his right knee for 3 days. He states that he often has to clear small nails or glass splinters from the rough pavement of his construction site before kneeling to do work-related tasks. He has never had a prosthetic implant, there is no family history of rheumatoid arthritis or gout, and that he does not have unexplained skin lesions. Examination shows there is an obvious effusion in his right knee. It is visibly red and swollen, and warm and painful to touch. He refuses to bend it. His left knee is unaffected.  Which of the following would be the most helpful test in establishing a diagnosis?