The Gram stain showing Gram-negative diplococci in cerebrospinal fluid (CSF) is characteristic of Neisseria meningitidis, a leading cause of bacterial meningitis in adolescents and young adults.

Step-by-Step Justification:

Gram Stain Interpretation:

Gram-negative diplococci in CSF strongly suggest Neisseria meningitidis​.

Classic Symptoms of Meningitis:

Fever, stiff neck, and vomiting are hallmark signs of meningococcal meningitis.

Neisseria meningitidis vs. Other Bacteria:

Haemophilus influenzae (Option A) → Gram-negative coccobacilli.

Listeria monocytogenes (Option C) → Gram-positive rods.

Streptococcus pneumoniae (Option D) → Gram-positive diplococci.

CBIC Infection Control References:

APIC Ready Reference for Microbes, "Neisseria meningitidis and Meningitis"​.