A 6-week-old full-term male is brought into the emergency room for nonbilious emesis. The mother reports that he began regurgitating breast milk 1 week ago. Although intermittent at first, nonbilious emesis now occurs after every feeding and has become progressively more forceful with increased volume. His appetite remains vigorous, even immediately after vomiting. He is the mother’s first child and his delivery was uneventful. His blood pressure is normal and he is afebrile but tachycardic. On physical examination, he is irritable and has a sunken fontanelle. A small, firm mass is palpated in the right upper quadrant.