A mother brings her 1-month-old infant to the pediatrician. She says the baby is crying more than usual and is vomiting and does not want to eat. Meningitis is suspected, and a lumbar puncture is done, which shows the following results; Opening pressure = 240 mm H2O (normal = 100-200 mm H2O), WBC count: 1200/mm3, Protein: 200 mg/dL, Glucose: 30 mg/dL, Gram stain: gram-positive rods, W hich of the following organisms is most likely responsible for this infant’s meningitis?

**Core Concept**
In this scenario, the infant presents with classic symptoms of meningitis, including irritability, vomiting, and refusal to feed, accompanied by abnormal cerebrospinal fluid (CSF) findings. The CSF analysis reveals elevated pressure, increased white blood cell (WBC) count, elevated protein, decreased glucose, and the presence of gram-positive rods on Gram stain.

**Why the Correct Answer is Right**
The presence of gram-positive rods in the CSF, coupled with the clinical presentation of meningitis, points towards a bacterial infection caused by Listeria monocytogenes. This organism is a common cause of meningitis in neonates and young infants, particularly in the first month of life. Listeria monocytogenes is a facultative intracellular pathogen that can cross the placental barrier and infect the fetus, leading to congenital infection. The bacteria can also be acquired postnatally through contaminated food or water.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a gram-positive cocci, not rods, making it an unlikely cause of meningitis in this scenario.
**Option B:** E. coli is a gram-negative rod, which does not match the Gram stain results.
**Option C:** Haemophilus influenzae is a gram-negative coccobacillus, not a gram-positive rod.

**Clinical Pearl / High-Yield Fact**
Listeria monocytogenes meningitis is a medical emergency that requires prompt treatment with intravenous antibiotics, such as ampicillin or penicillin, and supportive care. Early recognition and treatment can significantly improve outcomes in affected infants.

**Correct Answer:** D. Listeria monocytogenes

✓ Correct Answer: B. Listeria monocytogenes