A newborn male, born at 38 weeks gestation via spontaneous vaginal delivery, presents with persistent vomiting shortly after the first feed. The mother reports greenish-colored vomit and mild abdominal distension. The baby has not passed meconium since birth. There are no significant complications during pregnancy, but the mother mentions a prenatal ultrasound showing polyhydramnios. On physical examination, the baby appears slightly dehydrated but is otherwise alert and active. The abdomen is mildly distended, with no palpable masses or tenderness. Bowel sounds are present but reduced. A nasogastric tube is inserted, and bilious fluid is aspirated. You suspect duodenal atresia and decide to order some x-rays. What is the most characteristic radiological finding in duodenal atresia?
