An otherwise healthy 9-year-old girl is brought to the emergency department after the acute development of fever (38.8°C [101.8°F]), nausea, vomiting, diarrhea, and crampy periumbilical abdominal pain. A review of her diet in the past 48 hours includes cereals, fruits, egg salad sandwiches, steamed vegetables, hamburger, grilled chicken, Christmas cookies, and homemade eggnog. At the time of evaluation, her vital signs are stable but she has a mild fever of 38.4°C (101.1°F). She is fatigued
but alert. Her abdominal examination is notable for periumbilical tenderness, but no guarding or rebound tenderness is noted, and no masses are palpated. Select the most likely etiologic agent.