A 28-year-old woman from Delhi presents with right upper quadrant pain, fever (38.5°C), and hepatomegaly. Abdominal ultrasound shows a 6 cm hypoechoic lesion in the right lobe of the liver with a well-defined border. She reports a history of dysentery 2 months ago. Serology (IHA) is positive. Stool microscopy is negative for parasites. What is the most likely diagnosis?

Explanation

## Diagnosis: Amoebic Liver Abscess (Entamoeba histolytica) ### Clinical Presentation **Key Point:** Amoebic liver abscess (ALA) is the most common extraintestinal complication of *E. histolytica* infection. It typically develops 2–3 months after intestinal infection, often when the intestinal symptoms have resolved. ### Diagnostic Criteria Met | Criterion | Finding | Significance | |-----------|---------|---------------| | **Timeline** | Dysentery 2 months prior | Matches typical latency for ALA development | | **Geography** | Delhi (endemic zone) | E. histolytica is endemic in India | | **Serology (IHA)** | Positive | >90% sensitivity in ALA; >70% in intestinal disease | | **Stool microscopy** | Negative | Organism may not be shed during extraintestinal phase | | **Imaging** | Hypoechoic lesion, well-defined | Typical for amoebic abscess | | **Clinical signs** | RUQ pain, fever, hepatomegaly | Classic triad of ALA | ### Pathophysiology of Amoebic Liver Abscess 1. **Intestinal invasion:** Trophozoites penetrate colonic mucosa 2. **Portal circulation:** Organisms enter portal blood via mesenteric veins 3. **Hepatic seeding:** Trophozoites lodge in liver sinusoids, primarily right lobe (60–80% of cases) 4. **Abscess formation:** Cytotoxic enzymes destroy hepatocytes, creating a cavity 5. **Sterile pus:** Abscess contains acellular debris and trophozoites; bacterial superinfection is rare (unlike pyogenic abscess) **High-Yield:** The right lobe is preferentially affected because portal blood from the superior mesenteric vein (supplying the cecum and ascending colon — the primary site of intestinal invasion) drains directly to the right hepatic lobe. ### Imaging Characteristics **Mnemonic:** **AMOEBA** = **A**nterior/right lobe, **M**ultiple (can be solitary), **O**dd shape (irregular), **E**choic (hypoechoic or anechoic), **B**order (well-defined), **A**bscess (fluid collection). - **Ultrasound:** Hypoechoic or anechoic lesion with well-defined borders; may show internal echoes (granular debris) - **CT:** Low-attenuation lesion with rim enhancement after contrast - **MRI:** T2 hyperintense, T1 hypointense; rim enhancement ### Serology in Extraintestinal Disease **Clinical Pearl:** Serology (IHA, ELISA, or immunofluorescence) is highly sensitive (>90%) in amoebic liver abscess because the immune response is robust in extraintestinal disease. In contrast, serology is positive in only 70% of cases with intestinal infection alone. **Why stool is negative:** During the extraintestinal phase, trophozoites are sequestered in the liver and are not shed in feces. Stool microscopy becomes negative even though the patient has active disease. ### Differential Diagnosis: Why Other Options Are Wrong | Feature | Amoebic ALA | Pyogenic ALA | Hydatid Cyst | TB | |---------|---|---|---|---| | **Serology** | IHA positive | Blood culture positive | Serology (ELISA) positive | Negative | | **Stool** | May be negative | Negative | Negative | Negative | | **Pus character** | Sterile, acellular | Purulent, polymorphs | Clear fluid | Caseous | | **Imaging** | Hypoechoic, well-defined | Thick-walled, gas | Cystic, daughter cysts | Calcification | | **History** | Prior dysentery | Biliary/GI infection | Sheep/dog contact | TB elsewhere | ### Management of Amoebic Liver Abscess **Key Point:** Medical management is first-line; aspiration is reserved for diagnostic uncertainty or failed medical therapy. 1. **Tissue amebicide:** Metronidazole 750 mg TDS × 10 days (or tinidazole 600 mg BD × 5 days) 2. **Luminal amebicide:** Paromomycin 25–35 mg/kg/day × 7 days (to eradicate intestinal colonization) 3. **Aspiration:** Consider if: - Diagnosis uncertain (to confirm amebic etiology) - Large abscess (>5 cm) with risk of rupture - Failure to improve after 72 hours of medical therapy - Abscess in left lobe (risk of pericardial rupture) **Aspiration fluid characteristics:** - **Appearance:** "Anchovy paste" — chocolate-brown, acellular - **Culture:** Sterile (no bacteria) - **Microscopy:** Few/no WBCs; trophozoites may be seen at abscess margin - **PCR:** Can detect *E. histolytica* DNA [cite:Park 26e Ch 5, Harrison 21e Ch 227]