A 32-year-old man from rural Bihar presents with a 3-week history of bloody diarrhea, abdominal cramps, and tenesmus. He reports passing 6–8 stools daily with visible blood and mucus. Physical examination reveals mild abdominal tenderness over the cecum and ascending colon. Stool microscopy shows trophozoites with ingested RBCs. Serology for anti-Entamoeba antibodies is negative. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Invasive Amebic Colitis ### Key Clinical Features **Key Point:** The presence of trophozoites with ingested RBCs (erythrophagocytosis) in stool is pathognomonic for invasive disease, not asymptomatic colonization. **High-Yield:** Bloody diarrhea with mucus, tenesmus, and localized colonic tenderness in an endemic area strongly suggests invasive amebic colitis in its early stage. ### Distinguishing Invasive from Noninvasive Disease | Feature | Noninvasive (Luminal) | Invasive (Colitis) | Fulminant | |---------|----------------------|-------------------|----------| | **Stool findings** | Cysts only; trophozoites without RBCs | Trophozoites with RBCs | Trophozoites, blood, necrotic tissue | | **Serology** | Negative | Negative to weakly positive (early) | Positive (>90%) | | **Symptoms** | Asymptomatic or mild diarrhea | Bloody diarrhea, cramps, tenesmus | Severe, fever, shock, perforation risk | | **Duration** | Chronic, indolent | Subacute (1–4 weeks) | Acute, fulminant | | **Colonic findings** | Normal or minimal | Ulceration, flask-shaped ulcers | Extensive necrosis, toxic megacolon | ### Why This Is Early Invasive Disease (Not Fulminant) 1. **Serology is negative**: Fulminant colitis and liver abscess typically show positive serology (>90% of invasive cases); negative serology here suggests early mucosal invasion before strong antibody response. 2. **No systemic toxicity**: Fever, shock, or signs of perforation are absent — fulminant disease presents acutely with high fever and hemodynamic instability. 3. **Localized tenderness**: Cecal/ascending colon tenderness without diffuse peritonitis or rebound suggests active colitis without perforation or toxic megacolon. 4. **Erythrophagocytosis in trophozoites**: This is the hallmark of tissue invasion and rules out luminal colonization. **Clinical Pearl:** In endemic areas, the classic triad of bloody diarrhea + trophozoites with RBCs + negative serology = early invasive colitis. Serology becomes positive within 7–10 days of invasion as the immune response mounts. ### Pathophysiology ```mermaid flowchart TD A[Ingestion of E. histolytica cysts]:::outcome --> B[Excystation in colon] B --> C[Trophozoites colonize mucosa] C --> D{Virulence factors present?}:::decision D -->|No| E[Asymptomatic luminal colonization]:::outcome D -->|Yes| F[Mucosal invasion begins] F --> G[Erythrophagocytosis + bloody diarrhea]:::outcome G --> H{Invasion progresses?}:::decision H -->|Limited| I[Early invasive colitis]:::action H -->|Extensive| J[Fulminant colitis ± perforation]:::urgent I --> K[Serology becomes positive 7-10 days]:::action J --> K ``` **Mnemonic:** **RHINO** — RBC ingestion, Hemorrhagic diarrhea, Invasive trophozoites, Negative serology early, Organized colitis (not fulminant) [cite:Robbins 10e Ch 8]