Review Question - QID 109240

A 3-week-old boy presents to the emergency department with vomiting. His parents report that he suddenly started vomiting this morning and has shown no interest in feeding since then. They describe the vomitus as green and without any traces of blood. Prior to today, the patient was feeding, voiding, and stooling well. He was noted to have surpassed his birth weight at his last office visit 1 week ago. His temperature is 97.6°F (36.4°C), blood pressure is 78/56 mmHg, pulse is 148/min, and respirations are 44/min. On physical exam, the patient is in mild distress. He has no dysmorphic features, and his mucous membranes are dry. His abdomen is soft and distended. Bowel sounds are hypoactive. An abdominal radiograph is performed as in Figure A. Which of the following is the most appropriate next step in the diagnosis of this condition?