A 32-year-old man from rural Odisha presents with bloody diarrhea, abdominal pain, and fever for 10 days. Stool microscopy shows trophozoites with ingested RBCs. Which feature best distinguishes his intestinal infection from extraintestinal amoebiasis (liver abscess)?

## Intestinal vs. Extraintestinal Amoebiasis: Key Distinguishing Features ### Clinical Context This patient has **acute intestinal amoebiasis**, confirmed by the hallmark finding of RBC-ingesting (erythrophagocytic) trophozoites in stool. The question asks which feature **best distinguishes** his intestinal infection from extraintestinal amoebiasis (amoebic liver abscess). ### Key Discriminating Feature **Key Point:** The **presence of trophozoites with ingested RBCs in stool** is the cardinal feature of *intestinal* amoebiasis. In extraintestinal amoebiasis (amoebic liver abscess), trophozoites are typically **absent from stool**, making Option A the best distinguishing feature of the patient's intestinal disease. ### Comparative Table | Feature | Intestinal Amoebiasis | Extraintestinal (Liver Abscess) | | --- | --- | --- | | **Trophozoites in stool** | **Present** (RBC-ingesting) | **Absent** (or rare) | | Serology (IgG antibodies) | Positive (60–70%) | **Positive (>90%)** | | Clinical presentation | Acute dysentery, bloody diarrhea | Fever, hepatomegaly, RUQ pain | | Onset | Acute (days to weeks) | Subacute to chronic (weeks to months) | | Stool culture | Positive | Negative | ### Why Option A is Correct **High-Yield:** The presence of erythrophagocytic trophozoites in stool is **pathognomonic for active intestinal amoebiasis** (amoebic dysentery). This finding is characteristically *absent* in amoebic liver abscess, where the parasite has migrated via the portal circulation to the liver and the intestinal phase may have resolved. Therefore, trophozoites in stool are the feature that best distinguishes intestinal from extraintestinal disease. ### Why the Other Options Are Incorrect - **Option B (Elevated serum IgG):** Serology is positive in *both* intestinal and extraintestinal amoebiasis (though higher titers are seen in liver abscess >90%). It does not distinguish between the two forms. - **Option C (Acute onset with fever and bloody diarrhea):** This describes the patient's own presentation and is a feature of intestinal amoebiasis, but it is not a laboratory/diagnostic distinguishing feature and overlaps with the vignette itself. - **Option D (Absence of trophozoites in stool despite positive serology):** This pattern is characteristic of *extraintestinal* amoebiasis (liver abscess), not of the intestinal disease described in the stem. The question asks what distinguishes the patient's intestinal infection — not what characterizes liver abscess. **Clinical Pearl:** Per Harrison's Principles of Internal Medicine, the demonstration of *E. histolytica* trophozoites containing ingested erythrocytes in stool is diagnostic of invasive intestinal amoebiasis. In contrast, patients with amoebic liver abscess have positive serology but negative stool microscopy in the majority of cases. **Mnemonic:** **STOOL-SERUM rule** - **S**tool positive + Serum positive = Intestinal amoebiasis - **S**tool negative + Serum positive = Extraintestinal amoebiasis (liver abscess)