A 32-year-old man from rural Bihar presents with a 3-week history of bloody diarrhea with mucus, abdominal cramping, and tenesmus. He denies fever. Stool microscopy reveals trophozoites with ingested RBCs. Stool culture is negative for bacterial pathogens. Colonoscopy shows flask-shaped ulcers in the cecum and ascending colon with intact intervening mucosa. What is the most likely diagnosis?

Explanation

## Diagnosis: Entamoeba histolytica Intestinal Infection ### Clinical Presentation **Key Point:** The combination of bloody diarrhea with mucus, negative bacterial culture, and trophozoites containing ingested RBCs on stool microscopy is pathognomonic for *Entamoeba histolytica* infection. ### Diagnostic Features | Feature | E. histolytica | Shigella | V. cholerae | C. difficile | |---------|---|---|---|---| | **Stool appearance** | Bloody mucus (dysentery) | Bloody mucus | Rice-water stools | Watery ± blood | | **RBC ingestion** | Yes (trophozoites) | No | No | No | | **Flask-shaped ulcers** | Yes (cecum/colon) | Shallow ulcers | No ulceration | Pseudomembranes | | **Fever** | Usually absent | Present | Absent | Variable | | **Culture growth** | Not culturable (trophozoites) | Positive on culture | Positive on culture | Positive on culture | ### Microscopic Identification **High-Yield:** The hallmark of *E. histolytica* trophozoites is **erythrophagocytosis** — ingestion of RBCs within the cytoplasm. This is visible on wet mount and is diagnostic. **Mnemonic:** **HIST** = **H**istolytic (tissue-destroying), **I**ngests RBCs, **S**mall nucleus (4–20 μm), **T**rophozoites motile. ### Pathology of Intestinal Disease 1. Trophozoites invade the colonic mucosa and submucosa 2. Produce cytotoxic enzymes (serine proteases, phospholipases) 3. Create characteristic **flask-shaped ulcers** — narrow neck at mucosa, wide base in submucosa 4. Intervening mucosa remains intact (unlike bacterial dysentery) 5. Negative stool culture (organism is not culturable on routine media) ### Geographic Clue **Clinical Pearl:** Rural Bihar is an endemic zone for *E. histolytica* in India. Fecal-oral transmission via contaminated water is the primary route in areas with poor sanitation. ### Confirmatory Tests - **Stool microscopy:** Trophozoites with RBCs (repeat 3 samples on alternate days) - **Serology (IHA/ELISA):** Positive in invasive disease (>90% sensitivity in dysentery) - **Colonoscopy:** Flask-shaped ulcers with normal intervening mucosa - **Stool antigen detection:** PCR for *E. histolytica* (species-specific, differentiates from *E. dispar*) ### Treatment **Key Point:** Invasive intestinal disease requires both a luminal and tissue agent: - **Tissue agent:** Metronidazole 750 mg TDS × 10 days (or tinidazole) - **Luminal agent:** Paromomycin 25–35 mg/kg/day × 7 days (or diloxanide furoate) [cite:Park 26e Ch 5]