## **Core Concept**
The question describes a 1-week-old infant presenting with symptoms of vomiting, anorexia, fever, and a bulging fontanel. These symptoms suggest an infection affecting the central nervous system (CNS), specifically **bacterial meningitis**, which is a medical emergency.

## **Why the Correct Answer is Right**
The correct answer, **E. coli**, is a common cause of neonatal meningitis, especially in the first month of life. Neonatal meningitis can be caused by various pathogens, including Group B streptococcus (GBS), *Escherichia coli* (*E. coli*), and *Listeria monocytogenes*. The presentation of fever, vomiting, anorexia, and a bulging fontanel (indicative of increased intracranial pressure) is consistent with bacterial meningitis. *E. coli* is a gram-negative rod that can cause severe infections in newborns, including meningitis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain viruses can cause CNS infections, the symptoms described (particularly the bulging fontanel and the severity of systemic symptoms like fever and vomiting) are more indicative of a bacterial infection rather than a viral one.
- **Option B:** *Streptococcus pneumoniae* is a common cause of bacterial meningitis, but it typically affects older children and adults rather than neonates.
- **Option C:** *Listeria monocytogenes* can cause neonatal meningitis, but it is less common than *E. coli* or GBS in this age group.
- **Option D:** This option is not provided, but based on the context, we are focusing on why the other options are incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in neonatal meningitis, the classic triad of fever, vomiting, and altered mental status (which could be represented by a bulging fontanel in infants) is often present. Additionally, the most common causes of neonatal meningitis are GBS, *E. coli*, and *Listeria monocytogenes*. Early recognition and treatment are critical to prevent long-term neurological damage or death.

## **Correct Answer:** . *E. coli*.