You see a 9 month old girl with a 3 day history of vomiting, and episodes where she is screaming and goes pale in clear pain. Her mother has noticed she is drawing her legs up in discomfort and this is happening more often, every half hour at the minimum. There is no blood in her stool or history of recent travel. Her older brother had flu-like symptoms a week ago but is now better. The vomiting is now bile stained. Past medical history is unremarkable, and she was born at term with a normal vaginal delivery.
Based on the history alone what is the most likely working differential?