Hypertrophic pyloric stenosis: 

The patient's clinical presentation is a common one in pediatrics with a wide differential diagnosis. In this case, the sonographic findings of a hypertrophic pylorus with no passage of gastric contents clinches the diagnosis of hypertrophic pyloric stenosis (HPS). There is an additional bonus of having an upper GI series which demonstrates suggestive findings including an elongated narrow pyloric channel ("string sign") and pooling of contrast in the distal stomach. Most patients with abdominal complaints (pain, vomiting, etc.) will get a plain radiograph of the abdomen. Unfortunately, plain film is usually non-specific in hypertrophic pyloric stenosis but may be suggestive of it when a distended stomach and a paucity of distal bowel gas are identified.

In pediatric patients presenting with vomiting, the first question that you should ask is "bilious or not?". Unfortunately, differentiating non-bilious from bilious vomit is hard! So, it is not uncommon for patients with hypertrophic pyloric stenosis to undergo an upper GI study based on history from mother of yellowish appearing vomit in order to evaluate for possible midgut malrotation and volvulus.