A 7-year-old with bloody diarrhea who visited a petting zoo develops hemolytic uremic syndrome. The most likely etiologic agent is which organism?

• A. Escherichia coli 0157:H7
• B. Shigella dysenteriae
• C. Vibrio cholerae 01
• D. Vibrio cholerae non-O1

Answer: (A)

Shiga toxin–producing Escherichia coli infection from animal exposure is the classic cause of bloody diarrhea that can progress to hemolytic uremic syndrome in children. In this scenario, a child who had contact with a petting zoo is at risk for E. coli O157:H7, a Shiga-like toxin–producing strain that damages the lining of the colon, causing hemorrhagic colitis with bloody stools. The toxin also injures vascular endothelium, especially in the kidneys, leading to the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury seen in HUS. While Shigella can cause bloody diarrhea, it is not typically associated with petting zoos and HUS as commonly as E. coli O157:H7; cholera strains cause watery diarrhea rather than bloody stools, and non-O1 strains are not the typical culprits for HUS. Thus, the most likely agent is E. coli O157:H7.