Review Question - QID 5874

A 5-year-old boy is seen in the emergency room with a 3 day history of limping and painful range of motion of the left hip. He has no history of trauma. Observation of his gait demonstrates the ability to bear weight, although he has a notable limp. Radiographs are seen in Figures A and B. He is afebrile and his labs include: ESR of 20 mm/h (normal less than 22 mm/h), CRP of 18 mg/l (normal less than 20 mg/l), and WBC 9.0 (reference range 4.5 - 10.0). Which of the following most likely preceded the onset of his symptoms?