Entamoeba histolytica

Entamoeba histolytica is an intestinal protozoan parasite and the cause of amebiasis. It is transmitted by the fecal-oral route: ingested cysts from contaminated food or water survive stomach acid, release trophozoites in the intestine, and colonize the colon. Infection is most common in areas with poor sanitation and among travelers returning from endemic regions.

Many infections are asymptomatic, but invasive disease produces amebic dysentery — bloody diarrhea with mucus and abdominal pain. Trophozoites invade the colonic wall, creating the classic flask-shaped ulcers seen on histology. The organism can then spread through the portal circulation to the liver, forming an amebic liver abscess, typically a single lesion in the right lobe classically described as containing "anchovy paste" fluid, presenting with fever and right upper quadrant pain.

Diagnosis uses stool microscopy showing cysts (with up to four nuclei) or trophozoites — which may contain ingested red blood cells, a distinguishing feature — along with stool antigen testing and serology for invasive disease. Treatment pairs metronidazole (or tinidazole) to kill invasive trophozoites with an intraluminal agent such as paromomycin to eradicate remaining cysts.

USMLE Step 1 tests E. histolytica in parasitology and GI pathology. High-yield associations: fecal-oral cyst transmission, bloody diarrhea, flask-shaped colonic ulcers, right-lobe liver abscess, trophozoites with ingested RBCs, and metronidazole plus paromomycin therapy.