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?
Upper gastrointestinal contrast series
Contrast enema
Abdominal CT
Rectal suction biopsy
Upper gastrointestinal contrast series
This neonatal patient presents with sudden-onset bilious vomiting and an abdominal radiograph revealing a gasless abdomen, which is consistent with intestinal malrotation complicated by a midgut volvulus. The most appropriate next step in the diagnosis of intestinal malrotation is an upper gastrointestinal series.

Malrotation classically presents with bilious vomiting due to the formation of midgut volvulus. These patients will have normal development until the development of the obstruction followed by inability to feed and abdominal distention. Findings of malrotation on an upper gastrointestinal series include an abnormally placed ligament of Treitz on the right side of the abdomen and a "corkscrew" appearance of the distal duodenum. If an upper gastrointestinal contrast series confirms the diagnosis, patients should undergo surgery to reduce the volvulus in order to avoid intestinal perforation or other complications.

Adams and Stanton review the evidence regarding the diagnosis and treatment of neonatal bowel obstruction. They discuss how malrotation, jejunoileal atresia, duodenal atresia, and colonic atresia are common causes of this finding. They recommend surgical management of these conditions.

Figure/Illustration A is an abdominal radiograph demonstrating the nonspecific finding of a paucity of air in the abdomen (red circle). This is consistent with intestinal malrotation complicated by midgut volvulus.

Incorrect Answers:
Answer A: Although abdominal CT would detect intestinal malrotation, CT should be avoided in the pediatric population in light of the radiation exposure. A CT scan with oral contrast can be useful in the adult setting in order to determine if there is extravasation of fluid from the gastrointestinal tract.

Answer B: Abdominal ultrasound is the test of choice for diagnosing pyloric stenosis, which presents with non-bilious vomiting and a palpable olive mass in the abdomen. Treatment of this condition is with surgical release of the stenotic pyloric muscle.

Answer C: A contrast enema is used to differentiate between Hirschsprung disease and meconium ileus but would not be helpful in diagnosing malrotation as the midgut volvulus occurs in the small intestine. Both Hirschsprung disease and meconium ileus present in neonatal patients with bilious vomiting and failure to pass meconium in the first 48 hours of life. Treatment is with excision of the defective segment or removal of the meconium respectively.

Answer D: Rectal suction biopsy is the most accurate test to diagnose Hirschsprung disease. Although Hirschsprung disease commonly presents in neonatal patients with bilious vomiting, an abdominal radiograph in Hirschsprung disease would likely show dilation of the proximal colon.

Bullet Summary:
The most appropriate diagnostic test for malrotation in an infant is an upper gastrointestinal series, which will show the abnormal placement of the duodenum and ligament of Treitz on the right side of the abdomen.