Most common organism causing meningitis in a one year old child –
**Core Concept**
Meningitis is an infection of the meninges, the protective membranes surrounding the brain and spinal cord. In children under the age of 5, bacterial meningitis is a medical emergency that requires prompt treatment. The causative organisms can be classified into different age groups, with certain pathogens being more prevalent in infants and young children.
**Why the Correct Answer is Right**
In a one-year-old child, the most common cause of bacterial meningitis is *Streptococcus pneumoniae*. This pathogen is a gram-positive diplococcus that can invade the bloodstream and reach the central nervous system, causing meningitis. The bacteria can adhere to the epithelial cells of the respiratory tract and invade the bloodstream through the nasopharynx. The virulence of *S. pneumoniae* is attributed to its polysaccharide capsule, which helps the bacteria evade phagocytosis and establish infection.
**Why Each Wrong Option is Incorrect**
**Option A:** Escherichia coli (E. coli) is a gram-negative rod that can cause meningitis in neonates and young infants, but it is not the most common cause in one-year-old children.
**Option B:** Haemophilus influenzae type b (Hib) is a gram-negative coccobacillus that was a common cause of meningitis in children before the widespread use of Hib conjugate vaccines. However, vaccination has significantly reduced its incidence.
**Option C:** Group B streptococcus (GBS) is a gram-positive coccus that can cause meningitis in neonates and young infants, particularly those born to mothers with GBS colonization. However, it is not the most common cause in one-year-old children.
**Clinical Pearl / High-Yield Fact**
Vaccination is a critical component of preventing bacterial meningitis in children. The Hib conjugate vaccine has been highly effective in reducing the incidence of Hib meningitis, and the pneumococcal conjugate vaccine has also been shown to reduce the incidence of *S. pneumoniae* meningitis.
**Correct Answer:** C. Group B streptococcus (GBS)