Catherine brings her 6 month old son Michael for an emergency GP appointment. She is very worried as over the last 24 hours he has been crying when drawing his legs up several times an hour and vomited 6 times. This morning he had half his usual amount of milk and she also noticed some jelly-like stool in his nappy. He is having the usual number of wet nappies, has no rash or fever, and is not drowsy. He was born at term with normal vaginal delivery and is up to date with immunisations and is usually a well baby with no other medical conditions.
Based on the history you have a strong initial working diagnosis and commence examination.
What single examination finding would correlate with his most likely diagnosis?