A 6-year-old girl presents with abdominal pain, watery diarrhea that progressed to bloody diarrhea over the past two days, and nausea. She attended a family barbecue four days ago, where she consumed undercooked hamburgers. On examination, her temperature is 37.1°C (98.8°F), and she has mild dehydration. Which of the following is the most likely causative pathogen?