An 8-week-old male infant presents to the emergency department with 5 days of constipation and recent onset of fussiness, decreased feeding, and a swelling in the right neck. No temperature abnormalities are reported.

The medical history is significant for preterm birth at 30 5/7 weeks’ gestation (weight, 930 g) via cesarean delivery due to preeclampsia and partial placental abruption to a 33-year-old mother with an unknown group B Streptococcus (GBS) status. During his neonatal admission he received antibiotics for 48 hours after delivery for a sepsis evaluation and topical ciprofloxacin, which resolved an episode of undefined otorrhea. After an otherwise uneventful hospital course, he was discharged at age 6 weeks.

On examination in the emergency department, the vital signs are as follows: temperature, 97°F (36.1°C); heart rate, 172 beats/min; respirations, 40 breaths/min; blood pressure, 94/58 mm Hg; oxygen saturation, 100%; and weight, 2,400 g. The infant is in mild distress....

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