A 19-year-old male student presents to the emergency department with acute onset fever (39.8°C), severe headache, neck stiffness, and photophobia. Physical examination reveals a non-blanching petechial rash on the trunk and lower extremities. Blood cultures are pending. A Gram stain of cerebrospinal fluid shows Gram-negative kidney-bean diplococci that are oxidase-positive. The organism is identified as the species marked **C** in the diagram. Which of the following is the MOST important distinguishing biochemical feature of this organism that confirms the diagnosis?