Which of the following is the largest intestinal protozoa?
**Core Concept**
The question is testing knowledge of intestinal protozoa, which are single-celled parasites that infect the gastrointestinal tract. The correct answer requires identification of the largest species among the options provided.
**Why the Correct Answer is Right**
Entamoeba histolytica is a significant intestinal protozoan that causes amoebiasis, leading to symptoms such as diarrhea, abdominal pain, and bloody stools. This parasite is characterized by its large size, typically measuring 10-60 micrometers in diameter, making it the largest among the intestinal protozoa. Its pathogenicity is attributed to its ability to invade the intestinal mucosa and cause tissue destruction.
**Why Each Wrong Option is Incorrect**
**Option A:** Giardia lamblia is a smaller intestinal protozoan, measuring 9-21 micrometers in diameter, and is known for causing giardiasis, characterized by watery diarrhea and malabsorption.
**Option B:** Balantidium coli is a large ciliated protozoan, but it is not the largest intestinal protozoan, measuring approximately 50-200 micrometers in length.
**Option C:** Cryptosporidium species are small intestinal protozoa, measuring 4-6 micrometers in diameter, and are known for causing cryptosporidiosis, characterized by watery diarrhea.
**Clinical Pearl / High-Yield Fact**
Remember that Entamoeba histolytica is the only intestinal protozoan that can cause invasive disease, leading to amoebic liver abscess and extraintestinal disease.
**Correct Answer:** D. Entamoeba histolytica.