A 4-month-old boy with a history of intermittent vomiting, epigastric pain, and occasional hematemesis presents to the emergency department with 3 days of nonbloody, nonbilious vomiting and a distended abdomen. He is afebrile and well-appearing but has a mildly distended abdomen with absent bowel sounds. Abdominal X-ray reveals a double bubble sign. The infant is resuscitated with intravenous fluids, and a nasogastric tube relieves the distension and vomiting. What is the next step in management?
a) Gastrograffin upper gastrointestinal series
b) Gastroscopy
c) Contrast enema
d) Upper gastrointestinal series with small bowel follow-through