A 32-year-old student presents to the Emergency Department (ED) via ambulance with severe abdominal cramps, vomiting, and bloody diarrhoea. She reports that her symptoms began suddenly 24 hours ago. The diarrhoea is characterised by frequent, small-volume stools mixed with blood and mucus. She denies any recent travel, antibiotic use, or new foods prior to the onset of symptoms. She lives alone and has not been in contact with anyone known to have similar symptoms. She denies any history of inflammatory bowel disease or similar gastrointestinal issues. On examination, she is febrile with a temperature of 38.2°C. Her RR is 18 b/min, BP is 110/65 mmHg, and HR is 115 bpm. She appears dehydrated and in discomfort, with dry mucous membranes and poor skin turgor. Abdominal examination reveals diffuse tenderness, particularly in the lower quadrants, without rebound tenderness or guarding. Bowel sounds are hyperactive.

All the following could be the likely causative organism for her bloody diarrhoea EXCEPT?‎ *
Reveal the answer’s explanation.

► Dysentery is simply diarrhoea with blood – it is usually associated with fever but this is not always the case.

► Differentials include: –
  • Bacterial:
    • Gram-positive – Clostridium difficile
    • Gram negative – Shigellosis, Enterohaemorrhagic E.coli, Salmonella, Yersinia enterocolitica
  • Protozoa – Entamoeba histolytica, Balantidium coli
  • Helminths – Schistosoma (S. mansion or S. japonicum), Ascariasis, Trichuriasis
  • Non-infectious – inflammatory bowel disease, colorectal cancer, polyps, ischaemic colitis