A 6-year-old boy with homozygous sickle cell disease underwent splenectomy 4 weeks ago after recurrent vaso-occlusive crises. He presents to the emergency department with a 2-day history of high-grade fevers, productive cough, and right-sided pleuritic chest pain. On examination, his temperature is 39.6°C, pulse 120/min, and respiratory rate 28/min. Lung auscultation reveals decreased breath sounds and dullness to percussion over the right lower lobe. A chest radiograph confirms right lower lobe consolidation. Blood cultures grow optochin-sensitive, alpha-hemolytic, gram-positive diplococci that are surrounded by a thick polysaccharide layer. Which virulence factor of this organism is primarily responsible for its ability to evade phagocytosis and resist opsonization?