Organism Identification: Entamoeba histolytica
The organism is Entamoeba histolytica (option c), as cysts with 4 nuclei are pathognomonic for this parasite. 1
Diagnostic Reasoning
The key diagnostic feature here is the presence of cysts with 4 nuclei in the stool specimen. This morphologic finding is specific to Entamoeba histolytica and distinguishes it from the other organisms listed:
Giardia lamblia cysts contain 2-4 nuclei but are oval-shaped with a characteristic "falling leaf" motility of trophozoites; the mature cyst typically has 4 nuclei but appears morphologically distinct from E. histolytica 1
Trichomonas vaginalis is a urogenital pathogen, not an intestinal parasite, and does not form cysts—it exists only as motile trophozoites 1
Cryptosporidium produces small oocysts (4-6 micrometers), not cysts with visible nuclei, and requires special staining (modified acid-fast) or immunofluorescence for identification 1
Clinical Context
This patient's presentation of watery diarrhea after travel is consistent with E. histolytica infection, which is listed among the persistent/chronic diarrhea pathogens in travelers 1. The Infectious Diseases Society of America guidelines specifically identify E. histolytica as a cause of persistent diarrhea and note that travelers with diarrhea lasting 14 days or longer should be evaluated for intestinal parasitic infections 1.
Important Diagnostic Considerations
Microscopic examination remains the traditional diagnostic method, though it requires skilled technicians to differentiate E. histolytica from the non-pathogenic Entamoeba dispar, which is morphologically identical 1. The sensitivity of microscopic examination of a single stool specimen for detecting ova, cysts, and parasites generally exceeds 80%, with additional samples increasing sensitivity 1.
Antigen detection tests or PCR can distinguish pathogenic E. histolytica from E. dispar and have improved sensitivity and specificity compared to microscopy alone 1. Enzyme immunoassays for E. histolytica antigen detection are now available and recommended when microscopy is positive for Entamoeba species 1.
Clinical Significance
E. histolytica can cause both intestinal and extraintestinal disease. While this patient presents with watery diarrhea, the organism is also associated with visible blood in stool and can cause invasive colitis 1. The presence of fever would further suggest E. histolytica over other parasitic causes, as higher temperatures are suggestive of bacterial etiology or E. histolytica specifically 1.
Common Pitfall
Do not assume all Entamoeba cysts with 4 nuclei require treatment—E. dispar is morphologically identical but non-pathogenic 1. However, in a symptomatic traveler with diarrhea, the finding of quadrinucleate Entamoeba cysts should prompt treatment for E. histolytica while awaiting confirmatory antigen testing or PCR 1.