A 52-year-old man with a history of traumatic splenectomy presents to the emergency department with fever, chills, and hypotension. Blood cultures grow a gram-positive, alpha-hemolytic, optochin-sensitive, catalase-negative organism. Despite aggressive fluid resuscitation and empiric broad-spectrum antibiotics, the patient remains septic. Which feature of this organism is most directly responsible for its resistance to opsonization and phagocytic clearance in this asplenic patient?