The main symptom of the 3-hour-old boy is vomiting green emesis after each feeding.Green emesis （Bilious vomiting） indicates that something obstruct the intestine underneath sphincter of Oddi.So it couldn't be Pyloric Stenosis（Answer E）。
According to physical examination,we know his upper is distended,soft,without palpable masses,we can exclude intussusception（Answer C）， which is notable for a palpable right lower quadrant mass in the examination.
Next,the X-ray shows visiable air in the stomach and duodenum,which called 'double-bubble sign'.Because the position of upper gastrointestinal series is right ,so the possibility of malrotation with volvulus（Answer D），which presents rectal hemorrhage and peritonitis, is very low.Hirschsprung's disease（Answer B）can also be excluded because of no dilated proximal bowel and a narrowed distal segment presented in the barium enema.
Duodenal atresia（Answer A） is the most likely diagnosis.It presents with bilious vomiting following any feeding in the first few hours of life. Constipation often develops after meconium is passed. Physical examination is notable for abdominal distention of predominantly the upper abdomen.Polyhydramnios is often the earliest sign. Infants with Down's syndrome are at increased risk of having duodenal atresia. A double-bubble sign, air bubbles in the stomach and duodenum on x-ray of the abdomen, is diagnostic of duodenal atresia.
本病例由岚哥岚逸筠篁解析。
