Correct Answer: B. Entamoeba Histolytica
Entamoeba histolytica causes flask-shaped ulcers in the colon—this is the pathognomonic lesion that distinguishes amebic colitis from other parasitic infections. The flask shape arises from the parasite's unique mechanism of invasion: trophozoites secrete proteolytic enzymes that create a narrow neck of ulceration at the mucosal surface, then burrow deeply into the submucosa and muscularis, creating a wide base—hence the characteristic "flask" or "bottle" appearance on histology and endoscopy. The ulcers are typically found in the cecum and ascending colon, though they can extend throughout the colon. In India, where E. histolytica is endemic (especially in areas with poor sanitation), amebic colitis presents with bloody diarrhea, tenesmus, and abdominal pain. The trophozoites invade the intestinal wall, causing tissue necrosis and hemorrhage. Chronic infection can lead to amebic liver abscess, the most common extraintestinal manifestation. The diagnosis is confirmed by stool microscopy (cysts and trophozoites), serology (positive in >90% of invasive disease), and colonoscopy showing characteristic ulcers. Treatment follows Indian guidelines: metronidazole for invasive disease, followed by a luminal agent (paromomycin or iodoquinol) to eliminate intestinal colonization.
Why the other options are wrong
A. Entamoeba vermicularis — This is incorrect nomenclature—Enterobius vermicularis (not Entamoeba) is the pinworm, which causes perianal itching and anal fissures, not flask-shaped colonic ulcers. The confusion of name similarity is an NBE trap. Pinworms cause mechanical irritation, not invasive ulceration. C. Helicobacter pylori — While H. pylori causes peptic ulcer disease in the stomach and duodenum, it does not cause colonic ulcers or flask-shaped lesions. It is a gastric pathogen, not a colonic one. This option tests whether students confuse the site of infection and the morphology of ulcers across different GI pathogens. D. Giardia lamblia — Giardia lamblia causes acute watery diarrhea and malabsorption by adhering to the small intestinal mucosa, but does not invade tissue or produce ulcers. It affects the duodenum and jejunum, not the colon. The pathology is mucosal inflammation without the characteristic flask-shaped ulceration seen in amebic colitis.
High-Yield Facts
- Flask-shaped ulcers are pathognomonic for Entamoeba histolytica colitis—narrow neck at mucosa, wide base in submucosa.
- Cecum and ascending colon are the most common sites of amebic ulceration in the colon.
- Amebic liver abscess is the most common extraintestinal manifestation; occurs in ~10% of patients with intestinal amebiasis in India.
- Metronidazole is the first-line drug for invasive amebic disease; must be followed by luminal agents (paromomycin) to prevent relapse.
- Serology (IHA or ELISA) is positive in >90% of invasive amebiasis and >70% of amebic liver abscess; stool microscopy has low sensitivity in invasive disease.
- Trophozoites (motile, 15–20 μm) are seen in acute diarrhea; cysts (10–14 μm, 4 nuclei) are seen in chronic infection and asymptomatic carriers.
Mnemonics
*FLASK = E. histolytica* Form (flask-shaped), Large base (submucosa), Acute invasion, Submucosal burrow, Kills tissue (proteolytic enzymes). Use this when you see 'flask-shaped ulcer' in any question. Ameba Colitis Sites: CAP Cecum, Ascending colon, P*roximal colon. These are the classic locations where E. histolytica* causes ulcers; remember the right colon is most affected.
NBE Trap
NBE pairs "flask-shaped ulcer" with multiple parasites to test whether students confuse morphology with organism. The trap is the inclusion of Enterobius vermicularis (pinworm) under the misspelled name "Entamoeba vermicularis"—students who know pinworms cause anal symptoms may be lured into thinking a similar-sounding organism causes colonic ulcers.
Clinical Pearl
In endemic Indian settings (rural areas, slums with poor sanitation), a patient presenting with bloody diarrhea and right lower quadrant pain should raise suspicion for amebic colitis. Serology is your friend here—a positive IHA/ELISA combined with flask-shaped ulcers on colonoscopy clinches the diagnosis. Always follow metronidazole with paromomycin to avoid relapse from intestinal cysts.
_Reference: Robbins Ch. 8 (Infectious Diseases); Harrison Ch. 218 (Amebiasis); KD Tripathi Ch. 47 (Antiprotozoal Drugs)_