Final answer:
A barium swallow study is the recommended initial diagnostic step for a newborn with bilious or projectile emesis, as it effectively outlines the intestinal tract to help identify causes of obstruction, potentially followed by surgery consultation if needed.
Step-by-step explanation:
A barium swallow study is a key step in the treatment of a newborn with bilious or projectile emesis. This type of medical imaging uses a suspension of barium sulfate to coat the intestinal tract, which provides greater visual detail on an X-ray than traditional methods. It can help diagnose conditions like acid reflux disease, Crohn's disease, ulcers, and in the context of the question, potentially life-threatening conditions like intestinal malrotation or volvulus in a newborn. A barium swallow study is preferred initially over more invasive procedures like an upper GI endoscopy which is represented by Figure 18.7.3, and over a contrast-enhanced CT scan due to the lower radiation exposure and its adequacy in delineating the cause of the obstruction at a young age. Surgery consultation may follow if an abnormality requiring surgical intervention is identified through the barium swallow study.