The blood stained nappy

A 9‐day‐old breastfed boy presents to hospital with haematemesis. The parents bring a nappy with the blood‐stained vomitus. The baby examines normally, and the mother has no visible nipple cracking or visible blood in expressed milk from either breast. A drop of sodium hydroxide solution (NaOH) was placed on the baby's vomitus on the nappy (Fig. 1a) and the sample turned from red to brown. Control blood samples from one of the authors (EM) (Fig. 1b) and a drop of left over blood from another baby's blood test (Fig. 1c) were treated in the same way with NaOH. The adult sample turned brown and the baby sample remained red. What is the cause of the baby's haematemesis? What other tests are required? (Answer on page 837)