A 3-year-old boy with incomplete immunization is brought to the emergency department with sudden onset high fever, drooling, and difficulty breathing. He is sitting upright with neck extended and has inspiratory stridor. A lateral neck radiograph shows a swollen epiglottis (“thumbprint” sign). A nasopharyngeal swab grows small, pleomorphic, gram-negative coccobacilli that require both factor X (hemin) and factor V (NAD⁺) for growth. The organism’s ability to evade phagocytosis and complement-mediated opsonization is primarily due to which of the following virulence factors?