Hypertrophic pyloric stenosis:
The patient's clinical presentation is a common one in paediatrics with a wide differential diagnosis. In this case, the ultrasonographic findings of a hypertrophied pylorus with no passage of gastric contents cinches 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 HPS but may be suggestive of hypertrophic pyloric stenosis 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 "billious or not". Unfortunately, differentiating non-billious from billious vomit is hard! So, it is not uncommon for patients with HPS to undergo an Upper GI Study based on history from mother of yellow'ish appearing vomit in order to evaluate for possible midgut malrotation and volvulus.