## **Core Concept**
The question revolves around identifying the most common cause of meningitis in infants beyond the neonatal period (postneonatal period), presenting with symptoms such as fever, irritability, bulging fontanelles, and neck stiffness. Meningitis in this age group can be caused by various pathogens, including bacteria, viruses, and fungi, but the specific etiology varies by age.

## **Why the Correct Answer is Right**
The correct answer, **Group B Streptococcus (GBS)**, is a well-known cause of meningitis and sepsis in newborns and infants, particularly in the first few months of life. GBS is a gram-positive bacterium that commonly colonizes the gastrointestinal and genital tracts of healthy adults. In infants, especially those younger than 3 months, GBS can cause severe infections, including meningitis, pneumonia, and sepsis. The presentation with fever, irritability, bulging fontanelles, and neck stiffness in a 20-day-old infant is consistent with bacterial meningitis, for which GBS is a leading cause.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, other bacteria like *Escherichia coli*, *Listeria monocytogenes*, and *Streptococcus pneumoniae* could be considered. However, without the specific option, we focus on why the provided correct answer is the most appropriate.
- **Option B:** Similarly, this option is not specified, but common causes like *Haemophilus influenzae* type b (Hib) are less common in this age group due to vaccination.
- **Option C:** Without specifics, it's hard to address directly, but viral causes are more common in older children and typically present differently.
- **Option D:** This option is also unspecified, but given the clinical presentation, a bacterial cause like GBS is more likely than a viral or fungal cause in this context.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the common causes of bacterial meningitis vary by age: *Group B Streptococcus*, *Escherichia coli*, and *Listeria monocytogenes* are common in infants less than 3 months old. The introduction of vaccines has shifted the epidemiology, making *Streptococcus pneumoniae*, *Neisseria meningitidis*, and *Haemophilus influenzae* type b more common in older infants and children.

## **Correct Answer:** D. Group B Streptococcus.