A 35 year old man who recently traveled to a third world country develops chronic, severe dysentery. Colonoscopy demonstrates ulceration of the cecum, and a cecal biopsy reveals 15-to-40 micron amoebae with ingested erythrocytes and small nuclei with distinctive tiny central karyosomes. Which of the following organisms is the most likely culprit?

**Core Concept**
Entamoeba histolytica is a protozoan parasite that causes amoebiasis, a disease characterized by intestinal infection, leading to symptoms like dysentery. The parasite invades the intestinal mucosa, causing ulceration and tissue damage.

**Why the Correct Answer is Right**
Entamoeba histolytica is identified by the presence of ingested erythrocytes and small nuclei with tiny central karyosomes in the cecal biopsy. This is due to the parasite's invasive nature, where it adheres to the intestinal mucosa via a surface lectin, leading to the formation of pseudopodia and subsequent invasion of the colonic epithelium. The presence of erythrocytes within the amoeba's cytoplasm is a hallmark of invasive amoebiasis, as the parasite ingests red blood cells as a source of nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** Acanthamoeba sp is a free-living amoeba that can cause keratitis and encephalitis in humans, but it is not typically associated with intestinal infection or dysentery.

**Option B:** Balantidium coli is a large protozoan parasite that causes balantidiasis, characterized by diarrhea and intestinal infection, but it is not typically associated with the presence of ingested erythrocytes or the specific histopathological features seen in this patient.

**Option D:** Giardia lamblia is a flagellated protozoan parasite that causes giardiasis, characterized by diarrhea and intestinal infection, but it does not invade the intestinal mucosa or cause the same level of tissue damage as Entamoeba histolytica.

**Clinical Pearl / High-Yield Fact**
In cases of suspected amoebiasis, it is essential to perform a thorough travel history and to consider the possibility of invasive amoebiasis, particularly in patients with chronic, severe dysentery and a history of travel to endemic areas. A cecal biopsy with histopathological examination is a crucial diagnostic tool in this scenario.

✓ Correct Answer: C. Entamoeba histolytica

✓ Correct Answer: C. Entamoeba histolytica