The patient's history of bone marrow transplantation and the involvement of the entire gastrointestinal (GI) tract - stomach through to rectum - helps us to pause and consider systemic causes of GI tract inflammation. The major considerations with such diffuse involvement include vasculitis, eosinophilic gastroenteritis, and graft versus host disease (GvHD), with GvHD being the most likely cause and confirmed in this case.