A 19-year old F presents to the ED with a 3-day history of a swollen and hot R knee. She is previously healthy and on no medications. She feels well otherwise. On examination she has a large effusion which is red and hot. Her synovial fluid analysis reveals the following: WBC: 55 000 PMN: 92% Gram stain: negative Microscopy: negative. Which of the following represents the BEST management plan?