A 7-year-old girl develops bloody diarrhea and later hemolytic uremic syndrome after visiting a petting zoo with calves, lambs, and chickens. Which etiologic agent is most likely?

• A. E. coli O157:H7
• B. Shigella dysenteriae
• C. Vibrio cholerae 01
• D. Vibrio cholerae non01

Answer: (A)

This scenario tests recognition of a Shiga toxin–producing organism acquired from animals, leading to hemorrhagic diarrhea and later hemolytic uremic syndrome in a child. The classic culprits are enterohemorrhagic E. coli, especially E. coli O157:H7, which are commonly carried by cattle and can be transmitted at petting zoos. The Shiga-like toxin damages intestinal mucosa, causing bloody diarrhea, and also injures endothelial cells in small vessels, particularly in the kidneys, leading to the triad of hemolytic anemia, thrombocytopenia, and acute kidney injury seen in HUS. While Shigella can cause severe diarrhea, the animal exposure pattern here points more strongly to E. coli O157:H7. Vibrio cholerae causes watery, not bloody, diarrhea and isn’t associated with HUS.