A 19-year-old male student presents with sudden-onset fever (39.5°C), severe headache, neck stiffness, and a petechial rash on the trunk and extremities. Cerebrospinal fluid (CSF) analysis shows pleocytosis (WBC 850/μL, predominantly neutrophils), elevated protein (120 mg/dL), and low glucose (28 mg/dL). Gram stain of CSF reveals gram-negative diplococci. Which is the most specific investigation to confirm Neisseria meningitidis meningitis?