What is the "double bubble" AXR sign associated with in a neonate less than 12 hours old who presents with bilious vomiting?

The "double bubble" sign seen on abdominal X-ray (AXR) in a neonate less than 12 hours old who presents with bilious vomiting is indicative of duodenal atresia. This radiological finding is characterized by two distinct air-filled spaces: one in the stomach and the other in the duodenum. The presence of this sign suggests a blockage in the duodenum, preventing further passage of air and fluid beyond that point.

Duodenal atresia is often associated with other congenital anomalies, such as Down syndrome, and typically presents shortly after birth with symptoms of bilious vomiting due to the obstruction preventing normal gastric and duodenal passage. The double bubble sign most specifically corresponds to this condition as it reflects the anatomy and pathology of a complete obstruction at the duodenal level, hence the characteristic appearance.

Other conditions listed, while they may cause bilious vomiting in a neonate, do not produce the specific "double bubble" sign. For instance, intestinal malrotation might cause similar symptoms but typically would not present with the same distinct double bubble appearance on X-ray. Jejunal atresia or meconium ileus would often present differently on imaging and may not produce the double bubble sign. Thus