A 24-year-old university student presents with three days of migratory joint pain involving both knees and one wrist. He has a temperature of 38.1°C and a small number of painless pustules on his trunk; examination also shows tenderness along the flexor tendon sheath of one finger. The knee is swollen and should be aspirated, but he has no prosthetic material and no recent surgery. Which one of the following is the most likely causative organism?