Organism Identification: Entamoeba histolytica
The organism causing watery diarrhea with cysts containing 4 nuclei in the stool is Entamoeba histolytica (answer c). This is a definitive microscopic finding based on the characteristic morphology of the cyst stage.
Diagnostic Reasoning
Cyst morphology with 4 nuclei is pathognomonic for Entamoeba histolytica. 1 This distinguishes it from other intestinal parasites:
Giardia lamblia (option a) produces cysts with 2-4 nuclei but has an oval shape with a characteristic "falling leaf" motility of trophozoites, not the round cysts with 4 distinct nuclei seen in E. histolytica 1, 2
Trichomonas vaginalis (option b) is a urogenital pathogen, not an intestinal parasite, and does not form cysts 1
Cryptosporidium (option d) produces oocysts that are 4-6 micrometers in diameter and require special staining (modified acid-fast) for visualization, not the larger cysts with visible nuclei characteristic of E. histolytica 1
Clinical Context in Travelers
E. histolytica is specifically listed among persistent or chronic diarrhea pathogens in travelers, along with Cryptosporidium, Giardia lamblia, Cyclospora cayetanensis, and Cystoisospora belli. 1 The travel history is a critical epidemiologic clue, as E. histolytica is endemic in developing countries with poor sanitation. 1
Travelers with diarrhea lasting 14 days or longer should be evaluated for intestinal parasitic infections including E. histolytica. 1 The organism can cause visible blood in stool, though watery diarrhea is also a recognized presentation. 1
Diagnostic Confirmation
While microscopic identification of cysts with 4 nuclei is diagnostic, additional testing with enzyme immunoassay or antigen detection for E. histolytica increases sensitivity and helps distinguish pathogenic E. histolytica from non-pathogenic E. dispar, which has identical microscopic appearance. 1 Stool antigen testing has sensitivity and specificity exceeding 95% for E. histolytica. 1
Multiple stool specimens (typically 3 collected on different days) increase diagnostic sensitivity to over 80% when relying on microscopy alone. 1
Critical Management Considerations
E. histolytica requires specific antiparasitic treatment even in cases presenting with watery diarrhea, as untreated infection can progress to invasive disease with colitis, liver abscess, or extraintestinal complications. 1 This distinguishes it from self-limited viral or some bacterial causes of traveler's diarrhea.
The presence of fever with E. histolytica infection suggests invasive disease, as higher temperatures are suggestive of bacterial etiology or E. histolytica specifically among parasitic causes. 1