A 7-month-old male infant presents with two episodes of green emesis, lethargy, oliguria, and decreased stool output. The mother states he was a full-term baby with no prior illnesses or surgery. He fed normally for months until the day prior to presentation. His last normal bowel movement was 24 hours ago. His heart rate is slightly elevated. He is normotensive and afebrile. He is lethargic but otherwise has a normal physical examination. His abdomen is soft, nontender, and nondistended.