A 22-year-old male college student presents to the emergency department with a 2-day history of high fever (39.5°C), severe headache, neck stiffness, and photophobia. On examination, he has a petechial rash on the trunk and lower extremities. Blood culture and CSF analysis are pending. A Gram stain of CSF shows Gram-negative diplococci that are kidney or coffee-bean shaped. The patient is immediately started on empiric antibiotics. Which of the following is the most likely causative organism?