A 34-yeаr-оld mаn presents tо the primаry care clinic cоmplaining of watery-loose stools over the past few weeks. He complains of abdominal distension and a very low appetite. He denies any fever or blood in the stool. He says that he has observed that his stools appear greasy and float in the toilet. He has lost 6 pounds in the last month without trying. He has no relevant health history and denies any changes to his diet or travel. Upon review of his risk factors, the FNP identifies that the man lives on a farm with a well water source. Based on his history, which of the following is the most likely cause of his symptoms?