PLE-3262

A 2-year-old girl has had fever and bloody diarrhea for 10 days. A stool culture obtained 7 days ago grew Salmonella species sensitive to amoxicillin. A blood culture was negative. Despite beginning oral amoxicillin therapy 4 days ago, her diarrhea has persisted. Current examination shows no other abnormalities, except for a temperature of 38.6 C (101.5 F). Which of the following is the most likely explanation for the failure of amoxicillin to improve her symptoms?