A 12-day-old boy is brought to the emergency department by his parents due to poor feeding. The patient is exclusively breastfed but has been less interested in feeding since last night. He was born via scheduled repeat cesarean delivery. Maternal group B streptococcal screening was positive, but intrapartum antibiotic prophylaxis was deferred due to the planned cesarean delivery. The pregnancy was otherwise unremarkable, and the infant had an uncomplicated nursery stay. The patient's 3-year-old brother attends day care and has had rhinorrhea and cough this week. Temperature is 35.1 C (95.2 F). Physical examination shows a hypotonic, lethargic infant with a full anterior fontanelle. Initial laboratory studies reveal a leukocyte count of 2,000/mm³ with 20% bands. During the evaluation, he becomes apneic and requires intubation. Which of the following is the most likely cause of this patient's condition?

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11ec25c8_d6c4_ef6e_bc81_c7c71329a2d7_MD0003_00 This patient's hypothermia, lethargy, and leukopenia with elevated bands are concerning for neonatal sepsis, a systemic bacterial infection in infants age ≤28 days. His full fontanelle and apnea are also worrisome for meningitis. Group B Streptococcus (GBS) is the most common cause of neonatal sepsis and meningitis. Early-onset GBS disease (age <1 week) is transmitted vertically by exposure of the infant to the maternal genital tract or infected amniotic fluid (due to bacterial ascension) during labor and delivery. The incidence of early-onset disease has decreased due to universal screening of pregnant women and administration of intrapartum antibiotic prophylaxis (IAP) to GBS-colonized mothers. IAP is not necessary for planned cesarean delivery without labor or rupture of membranes because GBS transmission is low in such settings. However, late-onset GBS disease (age >1 week), as is likely seen in this patient, is horizontally transmitted (eg, person to person on unwashed hands) via exposure to colonized contacts. Therefore, IAP has not changed the incidence of late-onset GBS disease. This infant was likely exposed to GBS at home from his colonized mother or another household contact. (Choice B) Haemophilus influenzae type b can cause bacteremia and meningitis, as well as pneumonia and epiglottitis, in unvaccinated children. However, H influenzae is a rare cause of sepsis in newborns due to low prevalence and protective maternal antibodies. (Choice C) Neonatal sepsis and meningitis due to Listeria monocytogenes are clinically indistinguishable from GBS. However, L monocytogenes is far less common. Furthermore, because L monocytogenes is transmitted transplacentally, infection is typically early onset (age <1 week) and there is usually a maternal history of flulike symptoms or gastroenteritis, neither of which is seen in this patient. (Choices D and E) Neisseria meningitidis and Streptococcus pneumoniae are common causes of meningitis in children age >1 month but rare causes of sepsis or meningitis during the neonatal period. (Choice F) Symptomatic congenital toxoplasmosis presents at birth with chorioretinitis, hydrocephalus, jaundice, and hepatosplenomegaly, none of which are seen in this patient. Educational objective: Group B Streptococcus is the most common cause of neonatal sepsis and meningitis. Prevention of early-onset infection includes maternal screening before delivery and intrapartum antibiotic prophylaxis, but these measures do not decrease the risk of late-onset (age >1 week) infection. References: Streptococcus group B. (http://www.ncbi.nlm.nih.gov/pubmed/31985936)

A 12-Day-Old Boy Is Brought to the Emergency Department by His