A 12-year-old girl was playing soccer when she began to limp. She has pain in her right leg and right upper thigh. Her temperature is 38.9°C (102°F). X-ray of the femur reveals that the periosteum is eroded, suggestive of osteomyelitis. Blood culture yields gram-positive bacteria. The most likely etiologic agent is which of the following?

Staphylococcus aureus