A 14-year-old girl suffered from abdominal pain, nausea, bloody diarrhea, and fever. Stool specimens are collected and sent to the laboratory for bacteriologic and parasitologic examination. Bacterial...

A 14-year-old girl suffered from abdominal pain, nausea, bloody diarrhea, and fever. Stool specimens are collected and sent to the laboratory for bacteriologic and parasitologic examination. Bacterial cultures are negative for intestinal pathogens. The laboratory report reveals organisms with red blood cells inside them. The most likely causal agent is *