A 58-year-old man with a history of splenectomy presents to the emergency department with sudden onset of high-grade fevers, chills, and hypotension. Blood cultures reveal gram-positive, alpha-hemolytic, optochin-sensitive, lancet-shaped diplococci. Despite aggressive fluid resuscitation and broad-spectrum antibiotics, his condition rapidly progresses to septic shock. Which virulence factor of this organism most directly contributed to its ability to evade phagocytosis and complement-mediated clearance in this asplenic patient?