Which parasite is responsible for causing severe dysentery?

Entamoeba histolytica is the parasite known for causing severe dysentery, particularly amoebic dysentery. This organism infects the intestines and can lead to the formation of ulcers in the intestinal wall, resulting in symptoms such as bloody diarrhea, abdominal pain, and cramping. The pathogenesis involves the parasite's ability to invade the colonic epithelial cells and its production of enzymes that contribute to tissue destruction. Chronic infections can also lead to severe complications, including perforation of the intestinal wall and secondary infections.

In contrast, Cryptosporidium parvum primarily causes a profuse, watery diarrhea, but it is not typically associated with the severe dysentery that characterizes Entamoeba histolytica infections. Giardia lamblia causes giardiasis, which may lead to diarrhea and gastrointestinal discomfort, but it generally does not result in the bloody stools seen in severe dysentery. Trichuris trichiura, also known as whipworm, can cause abdominal pain and diarrhea, but it is not known for causing dysentery. Therefore, Entamoeba histolytica is the clear choice when discussing severe dysentery specifically.