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). Figure 1 Multiple ileal lesions with stigmata of recent bleed. Figure 2 Area of ulceration associated with atypical mononuclear infiltrate. Figure 3 Atypical mononuclear infiltrate composed of cells with enlarged, irregular nuclei containing variably prominent nucleoli. Figure 4 Atypical cells displayed cytoplasmic expression of myeloperoxidase. Question What is the diagnosis?

中文翻译：

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从肠道思考：一例不明的下消化道出血

临床表现 一名中年男子因直肠新鲜出血而入院，这与排便无关。他最近在血液科门诊接受了巨细胞性贫血的检查。会阴检查显示 1 级内痔，无出血迹象。初步实验室检查显示巨红细胞性贫血（血红蛋白 10.5 g/dL，正常 12.9–17.0 g/dL；平均红细胞血红蛋白 95.3 fL，正常 80.0–95.0 fL）。外周血涂片显示原始细胞、发育不良的中性粒细胞、有核红细胞和低颗粒血小板。在持续性新鲜直肠出血后，患者接受了乙状结肠镜检查和内痔的橡皮筋结扎。随着低血压的发展和血红蛋白显着下降至 4，出血持续存在。8 g/dL 需要输血和重症监护。重复内窥镜检查，包括末端回肠插管，显示无并发症的右侧憩室病。在显着出血期间进行的 CT 肠系膜血管造影显示回肠造影剂外渗，但肠系膜血管造影未成功，可能是由于出血暂时停止。出血随后复发，鉴于持续性出血没有明确来源，总共输注了 12 个单位的压实细胞，因此进行了剖腹探查、台式小肠镜（图 1）和小肠切除术。对标本进行了组织病理学检查（图 2-4）。图 1 多处回肠病灶伴近期出血病灶。图 2 与非典型单核浸润相关的溃疡面积。图 3 由细胞组成的非典型单核浸润，细胞核增大、不规则，核仁明显不同。图 4 非典型细胞显示髓过氧化物酶的细胞质表达。问题 诊断是什么？