A 2-week-old infant presents with bilious vomiting and abdominal obstruction. What is the most likely diagnosis?

The presentation of a 2-week-old infant with bilious vomiting and abdominal obstruction strongly suggests the diagnosis of malrotation and volvulus. Bilious vomiting indicates that there is obstruction beyond the level of the duodenum, which often occurs when there is a twisting (volvulus) of the intestines due to abnormal rotation during fetal development (malrotation). This is a surgical emergency, as it can lead to ischemia of the affected bowel.

In contrast, jejunal atresia often presents with isolated vomiting and abdominal distension without the classic biliousness until later, since it primarily involves a non-patent segment of the intestine rather than an obstructive twist. Hypertrophic pyloric stenosis typically features non-bilious projectile vomiting in infants between 3 to 12 weeks of life due to distal gastric outlet obstruction, not at 2 weeks of age as commonly associated with bilious outputs. Acute appendicitis is quite rare in infants and would usually present differently, often with right lower quadrant pain rather than bilious vomiting and severe signs of obstruction.

The clinical features described align closely with malrotation and volvulus, making it the most likely diagnosis in this scenario.