Q. A 35-year-old male was recently diagnosed with lymphoma and underwent hematopoietic stem cell transplantation from an HLA-matched sibling. The patient’s medical history is significant for non-hodgkin lymphoma, treated with chemotherapy and radiation therapy prior to stem cell transplantation. His HLA-matched sibling was identified as the donor for the transplantation, which was uneventful with engraftment was achieved on day 12. However, the patient was readmitted two weeks post-transplant with watery diarrhea and abdominal pain. The patient reported 10 to 12 liquid stools each day, and denied any fevers. Physical examination revealed tenderness in his abdomen and mild dehydration. You ordered a complete blood count, electrolytes, and stool cultures. The results showed an elevated white blood cell count and liver enzymes, and his stool cultures are still pending. Based on your concerns, you also consulted with a gastroenterologist, who subsequently performed a rectal biopsy (see image), which showed apoptosis of epithelial cells. Considering his history and physical findings, which of the following is the most likely cause of this patient’s symptoms?