A 4-year-old male has recently recovered from chicken pox. He now refuses to walk. He has been complaining of pain worsening over the past 3 or 4 days. The patient has had tactile fevers. There has been no known trauma. An exam is remarkable for a temperature of 38.3C, pain to palpation of the right knee, and increased warmth of the right distal thigh. There are no joint effusions and range of motion is normal. Which of the following bacteria is most likely responsible for this illness?