A 21-year-old receptionist comes to the clinic reporting frequent diarrhea. She states that the stools are very loose and there is some cramping beforehand. She states this has occurred on and off since she was in high school. She denies any nausea, vomiting, or blood in her stool. Occasionally she has periods of constipation but that is rare. She thinks the diarrhea is much worse when she is nervous. Her past medical history is not significant. She is single and a university student majoring in accounting. She smokes when she drinks alcohol but denies any illegal drugs. Both of her parents are healthy. Her entire physical examination is unremarkable. What cause of diarrhea is the most likely etiology