<s>[INST] context: Many organisms can cause meningitis in the neonate, including E coli, L monocytogenes, H influenzae, Gram-negative rods, group B and D streptococci, and coagulase-positive and coagulase-negative staphylococci. Statistically, the most likely cause in this case would be late-onset group B streptococci (GBS) but, in this case this child has Gram-positive rods, an indication of L monocytogenes. Early onset listeriosis usually presents in the first day and always by day 3; fetal distress is common. These children have rashes and hepatosplenomegaly on presentation and a history of maternal fever is common. Late onset listeria typically presents with purulent meningitis. Early onset GBS is seen in the first 7 days of life and is associated with maternal complications such as prolonged rupture and chorioamnionitis; late onset GBS occurs after 7 days of life and is not related to maternal issues but rather environmental exposures. As expected, the incidence of early onset GBS has been steadily dropping with maternal prophylaxis; the incidence of late onset GBS and listeriosis has remained unchanged during the same time period. Clinical manifestations of meningitis in neonates include lethargy, bulging fontanel, seizures, and nuchal rigidity. The diagnosis is made with examination and culture of the CSF. Treatment is begun while awaiting the results of the spinal fluid analysis. Appropriate initial antibiotic coverage must include activity against Gram-positive and Gram-negative organisms (ampicillin and gentamicin or ampicillin and cefotaxime). [/INST] question: A 16-day-old infant presents with fever, irritability, poor feeding, and a bulging fontanel. Spinal fluid demonstrates Gram-positive rods. Which of the following is the most likely diagnosis? | options: A: Listeria monocytogenes B: Group A streptococci C: Group B streptococci D: Streptococcus pneumoniae | answer: A </s>