Vomiting in neonates infants and toddlers ddx 2

A neonate presents with bilious emesis and abdominal distention. Prenatal ultrasound was significant for polyhydramnios. Which diagnostic test is most appropriate to confirm the suspected condition?

1: Abdominal CT
2: Upper gastrointestinal contrast study
3: Barium enema
4: Abdominal ultrasound