A 50-year-old woman complains of weakness, profuse watery diarrhea, and crampy abdominal pain. She reports a 10-lb weight loss. Her serum potassium is 2.8 mEq/L. Select the most likely diagnosis.

WDHA or vasoactive intestinal polypeptide (VIPoma) is characterized by voluminous diarrhea, 5 L or more daily, rich in potassium, which looks like watery tea. The diarrhea is secretory and if refractory to antidiarrheal agents. Patients are weak, with metabolic acidosis and hypokalemia. Octreotide decreases diarrhea volume. The pancreatic tumor should be excised. Secretory diarrhea also occurs in some patients with Zolliger-Ellison syndrome (ZES) and is the only complaint in less than 10% of ZES patients. More than 90% of ZES patients have peptic ulcer disease. Unlike the diarrhea associated with ZES, the diarrhea of WHDA continues with fasting and continuous nasogastric tub suctioning. 15% of patients with glucagonoma have diarrhea; glucagonoma is associated with migratory necrotizing dermatitis.