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?

The major determinant of virulence in H. influenzae is the presence of a capsule. There is no demonstrable exotoxin, and the role of endotoxin is unclear. While one would expect that IgA protease would inhibit local immunity, the role of this enzyme in pathogenesis is as yet unclear. Flagella production is not considered a virulence factor. See the table below for a comparison of gram-negative rods associated with the respiratory tract.GRAM-NEGATIVE RODS ASSOCIATED WITH THE RESPIRATORYTRACTSpeciesMajor DiseasesLaboratory DiagnosisFactors X and V Required for Vaccine GrowthVaccine availableProphylaxis for ContactsHaemophilus influenzaeMeningitisa; otitis media, sinusitis, pneumonia, epiglottitisCulture; capsular polysaccharide in serum or spinal fluid++RifampinBordetelh pertussisWhooping cough (pertussis)Fluorescent antibody on secretions; culture-+ErythromycinLegionella pneumophilaPneumoniaSerology; urinary antigen; culture--None