Thinking out of the : a case of obscure lower GI bleeding.

Clinical Presentation: A middle-aged man was admitted for episodes of fresh per-rectal bleeding, which were not associated with defecation. He was recently investigated for macrocytic anaemia in the outpatient haematology clinic. Examination of the perineum revealed grade 1 internal haemorrhoids with no signs of bleeding.Initial laboratory tests revealed macrocytic anaemia (haemoglobin 10.5 g/dL, normal 12.9-17.0  g/dL; mean corpuscular haemoglobin 95.3 fL, normal 80.0-95.0  fL). Peripheral blood film showing blasts, dysplastic neutrophils, nucleated red blood cells and hypogranular platelets.The patient underwent a sigmoidoscopy and rubber band ligation of the internal haemorrhoids after persistent fresh per-rectal bleeding. The bleeding persisted with the development of hypotension and a significant drop of haemoglobin to 4.8 g/dL requiring blood transfusions and intensive care monitoring. Repeated endoscopy, including intubation of the terminal ileum, revealed uncomplicated right-sided diverticulosis. CT mesenteric angiography performed during an episode of significant bleeding revealed extravasation of contrast in the ileum, but mesenteric angiography was unsuccessful, possibly due to a temporary cessation of bleeding. Bleeding subsequently recurred and in light of the persistent bleeding with no clear source and with a total of 12 units of packed cell transfused, exploratory laparotomy, on-table enteroscopy (figure 1) with small bowel resection was performed. Histopathological examination of the specimen was performed (figures 2-4).gutjnl;66/12/2068/GUTJNL2016313252F1F1GUTJNL2016313252F1Figure 1Multiple ileal lesions with stigmata of recent bleed.gutjnl;66/12/2068/GUTJNL2016313252F2F2GUTJNL2016313252F2Figure 2Area of ulceration associated with atypical mononuclear infiltrate.gutjnl;66/12/2068/GUTJNL2016313252F3F3GUTJNL2016313252F3Figure 3Atypical mononuclear infiltrate composed of cells with enlarged, irregular nuclei containing variably prominent nucleoli.gutjnl;66/12/2068/GUTJNL2016313252F4F4GUTJNL2016313252F4Figure 4Atypical cells displayed cytoplasmic expression of myeloperoxidase.

Question: What is the diagnosis?