Review Question - QID 218638

A 12-year-old boy presents with a four-day history of progressive pain and difficulty ambulating on his right leg. His parents state that he sustained a minor laceration to his right knee last week during a soccer game. Physical exam reveals a well-developed, uncomfortable-appearing child of expected appearance for his age. There is a 3cm healing laceration over the anterior aspect of the proximal right leg. There is an area of poorly demarcated erythema surrounding the wound. There is moderate tenderness to palpation and swelling about the proximal leg, but the patella is not ballottable and there is no palpable knee effusion. Lachmann, posterior drawer, and varus/valgus stress tests reveal a stable knee. There is some pain with calf squeeze. MRI findings are shown in Figures A and B. An ultrasound of the right leg is seen in Figure C. Which virulence factor is most responsible for these associated findings?