At your general medicine practice, your newest patient is a 2-year-old girl from Nigeria whose mother reports the child has excellent health, except for the diarrhea she has demonstrated. Diarrhea started within days of their arrival in the United States 3 weeks ago. History reveals nothing remarkable, but her mother comments on her love of (and continual consumption of) ice cream, something new to her since coming to this country. What could be causing her diarrhea?