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A 55-year-old man with a history of tobacco use presented with a 5-day history of diarrhea and rectal bleeding. This was associated with intermittent low-grade fever and weight loss that he was not able to quantify. There was no family history of malignancy. He had traveled frequently within South East Asia, and his last travel had been 6 months prior to this presentation. Physical examination revealed a medium-built male who was hemodynamically stable, with a temperature of 38°C on presentation. His cardiovascular, respiratory, and abdominal examination was normal, with no organomegaly. Digital rectal examination showed a craggy irregular nonobstructing mass at the 11-to 1-o'clock position, 6 cm from the anal verge, without contact bleeding. His laboratory investigations revealed a white blood cell count of 11.5 ϫ 10 9 /liter (neutrophils, 63%; lymphocytes, 24.5%; eosinophils, 3%), normocytic normochromic anemia (12.5 g/dl), and a platelet count of 373 ϫ 10 9 /liter. C-reactive protein level was

H istopathological examination of the rectal tissue revealed necrosis with inflammatory exudates and abundant amoebic trophozoites (see Fig. 1B to D in the photo quiz), confirming the diagnosis of Entamoeba histolytica infection. There was no evidence of malignancy. Entamoeba histolytica, a nonflagellated protozoan, exists in an environmentally resistant cyst stage and an infective trophozoite stage, which causes invasive disease. Infections occur when cysts are ingested, usually via contaminated food or water, such as during exposure to poor sanitary conditions or travel to developing countries. (1). HIV-positive patients and men who have sex with men may also acquire E. histolytica through sexual transmission via fecal-oral contact and, in rare cases, anal-genital intercourse (1-3). Ninety percent of infections are asymptomatic (4). Symptomatic disease comprises intestinal amebiasis and extraintestinal amebiasis, most commonly involving the liver (4). Amoebic colitis may occasionally manifest as amebomas, a mass of granulation tissue and chronic inflammation mimicking colon cancer, as seen in this patient (5). Key histopathological findings include the presence of amebas on the mucosal surface on intestinal mucosal biopsy specimens in an overlying inflammatory exudate. A large, flask-shaped mucosal ulceration may develop over time. The infective trophozoites may be substantially larger (10 to 60 m) than macrophages, have a punctate central karyosome, and display foamy amphophilic cytoplasm with ingested erythrocytes. Cysts are not seen in tissue. The presence of tissue invasion of infective trophozoites is characteristic of E. histolytica infection (6-8). The PAS-D stain enhances recognition of the amebas. The other diagnostic methods for amebic colitis are stool microscopy and antigen detection, PCR, and serology (9). Stool microscopy has limited sensitivity and specificity (less than 60%). Erythrophagocytosis is considered pathognomonic of E. histolytica infection for morphological stool microscopic diagnosis and may be used to differentiate this organism from Entamoeba dispar. However, erythrophagocytosis (as seen in Fig. 1C in the photo quiz) is not necessary for diagnosis when there is evidence of tissue invasion or extraintestinal infection. (9) Stool antigen detection has excellent (Ͼ95%) sensitivity and specificity, and some assays can differentiate between E. histolytica and E. dispar. PCR offers similar specificity (Ͼ90%) and moderate sensitivity (Ͼ70%) but is more costly and time-consuming to perform than stool antigen detection. Serology is less useful for diagnosis in patients who live in or frequently travel to regions of high endemicity, as it may be difficult to differentiate between recent and past infections. (9) This patient had a negative stool ova-and-parasite examination result on one sample.