Amoebic dysentery — DTM&H MCQ

Correct answer: B — Stool antigen or PCR for Entamoeba histolytica

The correct answer is B, stool antigen or PCR for Entamoeba histolytica. Light microscopy cannot distinguish the pathogenic species E. histolytica from morphologically identical, non-pathogenic Entamoeba dispar and Entamoeba moshkovskii, so a microscopy report of Entamoeba cysts should be labelled E. histolytica/E. dispar rather than treated as a definitive diagnosis. UK Standards for Microbiology Investigations recommend that antigen detection by enzyme immunoassay or DNA detection by PCR be used to differentiate these species and confirm invasive potential before committing to a diagnosis of amoebiasis. In a returning traveller with bloody diarrhoea after 3 weeks in rural India, this molecular or antigen-based confirmation is the key next step to guide targeted treatment with metronidazole plus a luminal agent, and to avoid unnecessary treatment of non-pathogenic carriage. Why the other options are wrong: A. Urine antigen for Legionella: this detects Legionella pneumophila causing atypical pneumonia, which is irrelevant to bloody diarrhoea and a positive amoebic-looking stool microscopy result. E. Schistosoma serology as the first test: schistosomiasis typically presents with haematuria, eosinophilia or hepatosplenic disease after freshwater exposure, and serology does not clarify the ambiguous Entamoeba cyst report from this stool sample. D. Serum Widal test: this outdated and unreliable serological test targets typhoid (Salmonella Typhi), not amoebic colitis, and has poor sensitivity and specificity even in typhoid diagnosis. C. Blood film for malaria parasites: malaria causes fever with haemolysis or thrombocytopenia, not bloody diarrhoea, and does not address the need to speciate the Entamoeba cysts already seen. Key point: microscopy showing Entamoeba cysts must be reported as E. histolytica/E. dispar and confirmed by antigen testing or PCR, since only E. histolytica is invasive and treatable disease should never be assumed from morphology alone.

Reference: UK Health Security Agency, UK Standards for Microbiology Investigations B 31: Investigation of specimens other than blood for parasites (2025), section on Entamoeba histolytica/E. dispar differentiation, https://www.rcpath.org/static/0cedf934-957f-4924-a083372352aacd86/UK-SMI-B-31i52-Investigation-of-specimens-other-than-blood-for-parasites-October-2025.pdf