A 38-year-old man with advanced HIV infection (CD4 count: 60 cells/mm³) presents with chronic watery diarrhea for 2 months, significant weight loss, intermittent fever, and fatigue. Colonoscopy is performed, and biopsy from the colonic mucosa demonstrates trophozoites with RBCs. What is the most likely causative organism?

1. A

   A. Giardia lamblia

2. B

   B. Cryptococcus neoformans

3. C

   C. Cryptosporidium parvum

4. D

   D. Entamoeba histolytica

Appeared in: INI-CET EXAM -2026

Explanation

• The key diagnostic finding is the presence of trophozoites containing ingested red blood cells (RBCs) in the colonic biopsy.
• This process, known as erythrophagocytosis, is the pathognomonic (uniquely characteristic) feature of invasive amebiasis caused by Entamoeba histolytica.
• The organism's ability to lyse tissue explains the patient's symptoms of chronic diarrhea, fever, and weight loss.
• The patient's severely immunocompromised state (CD4 count of 60) makes them highly susceptible to such opportunistic infections.

Why Other Options Were Wrong

• Option A: Giardia lamblia is a non-invasive parasite. It adheres to the surface of the small intestine but does not invade the mucosa or ingest red blood cells.
• Option B: Cryptococcus neoformans is a fungus, not a protozoan. It does not cause colitis. In HIV patients, it is a well-known cause of meningitis and pneumonia.
• Option C: While Cryptosporidium parvum is a major cause of chronic watery diarrhea in AIDS patients, it does not invade tissue or ingest RBCs. Diagnosis relies on identifying oocysts on the intestinal surface, typically with an acid-fast stain.

Related Visual

Clinical Relevance

• Nursing practice connection: Use the key finding related to Differential diagnosis of infectious diarrhea in an immunocompromised patient based on histopathological findings to guide bedside assessment, documentation, and the next nursing action.
• In severely immunocompromised patients (e.g., advanced HIV), diarrhea can be caused by a wide range of organisms, and a definitive diagnosis through biopsy or specific stool tests is critical for effective treatment.
• Accurate identification of E. histolytica is crucial because it requires specific anti-amebic therapy (e.g., metronidazole) to eradicate the invasive parasite and prevent life-threatening complications like liver abscess, peritonitis, or toxic megacolon.
• Nurses play a key role in collecting appropriate specimens (stool for ova and parasites, assisting with endoscopy) and monitoring the patient's response to treatment, including resolution of diarrhea and fever.

How to Approach the Question

• First, analyze the patient's profile: A 38-year-old man with advanced HIV and a very low CD4 count (60 cells/mm3). This immediately signals a high risk for severe opportunistic infections.
• Next, identify the primary symptoms: Chronic watery diarrhea, weight loss, and fever, which are common in many GI infections in this population.
• Focus on the single most definitive piece of information provided: The colonoscopy biopsy result showing 'trophozoites with RBCs'. This is a specific, pathognomonic laboratory finding.
• Systematically evaluate each option against this key finding.
• Recall or deduce that the ingestion of red blood cells (erythrophagocytosis) by a protozoan trophozoite is the classic hallmark of invasive Entamoeba histolytica.
• Eliminate the other options based on their known characteristics: Giardia is non-invasive, Cryptococcus is a fungus causing different diseases, and Cryptosporidium presents as oocysts, not RBC-ingesting trophozoites.

Concept Tested & Keywords

• Concept Tested: Differential diagnosis of infectious diarrhea in an immunocompromised patient based on histopathological findings.
• Stem keywords: advanced HIV infection, CD4 count, chronic watery diarrhea, colonoscopy, biopsy, trophozoites with RBCs
• Lead-in keywords: most likely causative organism
• Clinical cues: CD4 count: 60 cells/mm3 (indicates severe immunodeficiency)
• Clinical cues: Trophozoites with RBCs (pathognomonic finding for Entamoeba histolytica)

Question ID

QTe6DQ5OkmFRahpGPwrNky

Copy ID

Reference Book

E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 798-800

E6 Medicine Harrison 22e Part 1 p. 1640-1642