A four-week-old infant born at 39 weeks’ gestation, exclusively breast fed and has been healthy was brought in for an evaluation following a 2 day history of projectile vomiting that occurs after each feeding without with increased fussiness. The child's father mentioned that the baby appears to be without distress after vomiting and wants to feed immediately afterwards. Parents deny the infant has had fever, diarrhea or skin lesion; in addition, has not had exposure to individuals with simil...