**Question:** A 65-year-old man presents with fever, severe headache, and nuchal rigidity. Physical examination in the emergency department shows a Glasgow coma score of 7. Lumbar puncture reveals cloudy cerebrospinal fluid (CSF) with 1200 neutrophils/mm3, elevated protein, and decreased glucose. Which of the following is the most probable etiologic agent of this condition?

A. Streptococcus pneumoniae
B. Neisseria meningitidis
C. Escherichia coli
D. Staphylococcus aureus

**Core Concept:**
Neuroinvasive bacterial infections are a group of diseases caused by bacteria that can invade the central nervous system (CNS), leading to meningitis (inflammation of the protective membranes covering the brain and spinal cord) and/or meningoencephalitis (inflammation of both the membranes and the brain). These infections can cause fever, headache, neck stiffness, and impaired consciousness. The cerebrospinal fluid (CSF) analysis typically reveals pleocytosis (increased white blood cell count) with neutrophils, elevated protein, and decreased glucose levels.

**Why the Correct Answer is B:**

Neisseria meningitidis, often called meningococcus, is a gram-negative diplococcus bacterium responsible for meningococcal meningitis. In the given scenario, the presence of severe headache, fever, and elevated CSF neutrophil count (1200 neutrophils/mm3) are indicative of a bacterial meningitis. Among the options provided, Neisseria meningitidis (option B) is the most probable etiologic agent because it is a common cause of meningitis, particularly in immunocompetent individuals.

**Why Other Options are Incorrect:**

A. Streptococcus pneumoniae (option A) is another common cause of bacterial meningitis but is less likely due to the absence of fever and elevated CSF neutrophil count (1200 neutrophils/mm3) in this case.

C. Escherichia coli (option C) is less likely because it is typically associated with neonatal sepsis and is less likely to cause meningitis in adults.

D. Staphylococcus aureus (option D) is less likely due to the presence of fever and elevated CSF neutrophil count (1200 neutrophils/mm3) in this case. S. aureus is more commonly associated with meningitis in immunocompromised individuals.

**Why Option B is the Correct Answer:**

Neisseria meningitidis is a common cause of bacterial meningitis, particularly in immunocompetent individuals. The clinical presentation, including fever and elevated CSF neutrophil count (1200 neutrophils/mm3), is consistent with meningitis caused by this bacterium.

**Core Concept:**

Neisseria meningitidis is a gram-negative diplococcus bacterium that can cause meningococcal meningitis, which is a severe form of bacterial meningitis.

**Why Other Options are Incorrect:**