Correct Answer: A - The clinical presentation of a sudden, severe headache, photophobia, neck stiffness, and an elevated opening pressure on lumbar puncture is suggestive of acute bacterial meningitis. Neisseria meningitidis is a common cause of bacterial meningitis in adolescents and young adults. The presence of Gram-negative diplococci on Gram stain further supports this diagnosis.

B. Streptococcus pneumoniae
While Streptococcus pneumoniae is a common cause of bacterial meningitis in adults, it typically presents with a more gradual onset. The Gram stain would reveal Gram-positive cocci.

C. Haemophilus influenzae
Haemophilus influenzae, particularly type B, was previously a common cause of bacterial meningitis in children. However, the incidence has decreased significantly with the introduction of the Hib vaccine.

D. Escherichia coli
Escherichia coli is a common cause of neonatal meningitis but is less common in adults. The Gram stain would reveal Gram-negative rods.

E. Listeria monocytogenes
Listeria monocytogenes is more commonly associated with meningitis in immunocompromised individuals, the elderly, and pregnant women. It often presents with a more indolent course and may involve brainstem nuclei, leading to cranial nerve abnormalities. The Gram stain would show Gram-positive rods.