A 34-yeаr-оld wоmаn presents tо the primаry care clinic because she has been losing weight and having watery loose stools over the past few weeks. She complains of abdominal bloating, flatulence, and reports little to no appetite. She denies any fever or blood in her stool. She also observed that her stools appear greasy and float in the toilet. She has no relevant medical history. She has not made any changes to her diet. She has not traveled outside the country. She does live on a large farm with a well for the water source. Based on her history, which of the following is most likely the cause of her symptoms?