A 2-month-old boy, who had been born at term, presents to the emergency department with irritability and poor feeding. The infant had been healthy with no temperature instability until approximately 2 weeks ago, when he became fussier than usual. Yesterday he refused to breastfeed and had a decreased number of wet diapers. His last stool was 5 days ago. His parents report that the infant is drooling more and appeared to choke on his secretions today.

The pregnancy was unremarkable and was completed with a normal vaginal delivery. The infant is exclusively breastfed, with good weight gain and normal development. Family members were ill with a stomach virus 10 days ago, but the infant appeared unaffected. His only medications are probiotics, chamomile, and an herbal supplement. Recent travel includes camping along the coastline. The infant has not received any vaccines.

Vital signs are as follows: temperature of 36.7°C (98.1°F), heart...

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