A 2-year-old boy who missed several scheduled immunizations presents to the emergency room with a high fever, irritability, and a stiff neck. Fluid from a spinal tap reveals 20,000 white blood cells per milliliter with 85% polymorphonuclear cells. Gram stain evaluation of the fluid reveals small pleomorphic gram-negative rods that grow on chocolate agar. If an inhibitor is designed to block its major virulence, which of the following would be the most likely major virulence factor?