**Core Concept**
Pyogenic meningitis is a bacterial infection of the meninges, the protective membranes surrounding the brain and spinal cord. In children, the age-specific epidemiology of pyogenic meningitis varies, with different pathogens more commonly affecting infants and young children.

**Why the Correct Answer is Right**
Neisseria meningitidis is a Gram-negative bacterium that commonly causes meningitis in children between 6 months and 2 years of age. This bacterium can lead to severe infections, including meningitis, septicemia, and pneumonia. N. meningitidis invades the bloodstream and crosses the blood-brain barrier, causing inflammation of the meninges. The capsule of N. meningitidis is a key virulence factor, allowing the bacterium to evade host immune responses.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus pneumoniae is a common cause of bacterial meningitis, but it is more commonly seen in children older than 2 years and in adults.
* **Option B:** Haemophilus influenzae type b (Hib) was a leading cause of bacterial meningitis in young children before the widespread use of Hib conjugate vaccines, but its incidence has significantly decreased in countries with high vaccination coverage.
* **Option D:** Escherichia coli is a rare cause of meningitis in children and is more commonly seen in neonates and individuals with underlying medical conditions.

**Clinical Pearl / High-Yield Fact**
The conjugate Hib vaccine has been highly effective in reducing the incidence of Hib disease, including meningitis, in countries with widespread vaccination coverage. This highlights the importance of vaccination in preventing bacterial meningitis in young children.

**Correct Answer: C. Neisseria meningitidis**