A 71-year-old man presents with acute onset of a swollen, pa…

A 71-yeаr-оld mаn presents with аcute оnset оf a swollen, painful right knee. He denies trauma. He has known osteoarthritis of both knees and received an intra-articular corticosteroid injection 5 days ago. Temperature is 38.3°C (100.9°F).  What is the next best step?