A 32-year-old man from rural Bihar presents with a 3-week history of bloody diarrhea (6–8 stools daily), abdominal cramping, and fever (38.5°C). He denies recent travel outside his village. Stool microscopy shows motile trophozoites with ingested RBCs. Stool culture is negative for bacterial pathogens. Abdominal ultrasound reveals a 4 cm hypoechoic lesion in the right lobe of the liver with a "target sign." What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Amoebic Liver Abscess ### Key Clinical Features **Key Point:** The combination of bloody diarrhea with trophozoites containing RBCs (erythrophagocytosis) + hepatic abscess with target sign is pathognomonic for invasive *Entamoeba histolytica*. ### Pathogenesis 1. Ingestion of *E. histolytica* cysts via fecal-oral route (endemic area) 2. Excystation in terminal ileum → trophozoite colonization 3. Invasion of colonic mucosa → mucosal ulceration and bloody diarrhea 4. Trophozoites penetrate submucosa → portal vein dissemination 5. Seeding in liver (right lobe > left due to blood flow) → abscess formation **High-Yield:** Erythrophagocytic trophozoites (RBC ingestion) in stool are diagnostic of invasive disease, not just colonization. ### Diagnostic Criteria Met | Feature | Finding | Significance | |---------|---------|---------------| | **Stool microscopy** | Motile trophozoites with RBCs | Invasive amebiasis (not cyst-passer) | | **Stool culture** | Negative for bacteria | Excludes bacterial dysentery | | **Ultrasound** | 4 cm hypoechoic lesion, target sign | Classic amoebic abscess appearance | | **Right lobe involvement** | Yes | Follows portal vein anatomy | | **Fever + diarrhea** | Present | Intestinal invasion + systemic response | ### Imaging Characteristics of Amoebic Abscess **Clinical Pearl:** The "target sign" (hyperechoic rim + hypoechoic center) reflects: - Outer rim: granulation tissue + inflammatory response - Center: liquefied necrotic debris (sterile, no bacteria) - Absence of gas or septations (unlike pyogenic abscess) ### Diagnostic Confirmation - **Serology (IHA/ELISA):** ~90% sensitive in amoebic liver abscess - **PCR:** Detects *E. histolytica* DNA in stool/abscess fluid - **Abscess aspiration:** Anchovy paste–like material (sterile on culture) **Mnemonic:** **AMOEBA** = **A**bscess in **M**iddle/right lobe, **O**utcome sterile, **E**rythrophagocytic trophozoites, **B**loody diarrhea, **A**nchovy paste drainage. [cite:Park 26e Ch 8]