The most common organism causing meningitis is –
**Core Concept**
Meningitis is an inflammation of the protective membranes covering the brain and spinal cord, known as the meninges. The most common cause of meningitis varies by age group, but overall, it is often due to bacterial pathogens. **Streptococcus pneumoniae** is a leading cause of bacterial meningitis.
**Why the Correct Answer is Right**
**Streptococcus pneumoniae** is a gram-positive coccus that can colonize the upper respiratory tract and, under certain conditions, invade the bloodstream and cross the blood-brain barrier to cause meningitis. Its polysaccharide capsule is a key virulence factor, helping the bacterium evade phagocytosis.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while **Haemophilus influenzae** type b (Hib) can cause meningitis, vaccination has significantly reduced its incidence.
**Option B:** Incorrect as **Neisseria meningitidis** causes meningitis, but it is less common than **Streptococcus pneumoniae** in many populations.
**Option D:** Incorrect because **Escherichia coli** can cause meningitis, especially in neonates, but it is not the most common cause overall.
**Clinical Pearl / High-Yield Fact**
Remember that the introduction of conjugate vaccines has altered the epidemiology of bacterial meningitis, with **Streptococcus pneumoniae** now being a predominant cause in many areas, particularly among adults and older children.
**Correct Answer:** D. Streptococcus pneumoniae