A 22-year-old male presents to the emergency department with a 12-hour history of severe headache, high fever (39.8°C), and neck stiffness. On examination, he has a positive Kernig sign and Brudzinski sign. A petechial rash is noted on his trunk and lower extremities. CSF analysis shows: WBC 450/μL (90% neutrophils), protein 180 mg/dL, glucose 25 mg/dL (serum glucose 95 mg/dL). Gram stain of CSF shows gram-negative diplococci. Which of the following is the most appropriate initial antibiotic therapy?