A 40 year old woman presents with months of diarrhea. Even when fasting, the stool volume is >700 mL/day; it averages 3 L/day. You hate to ask, but you learn the stool is tea-colored and odorless. There is no abdominal pain, but she gets some flushing, lethargy, nausea, vomiting, muscle weakness, and muscle cramps. Labs show a low potassium. A stool analysis shows an osmotic gap of 30 mOsm/kg (normal 50-125).
Challenge: What's the diagnosis?