A 30 year-old female presents with a one to two year history of daily bouts of mid-abdominal crampy pain and bloating that are briefly relieved with defecation. Loose to watery bowel movements occur four to five times daily. She denies any nocturnal symptoms. Previous laboratory studies, abdominal CT scans and a colonoscopy have all been normal. Attempted dietary changes, exercise and probiotics have provided no relief. What is the next step in treatment for this patient? A. Nortriptyline (Pamelor) B. Omeprazole (Prilosec) C. Polyethylene glycol (Miralax) D. Promethazine (Phenergan)