A 34-year-old woman comes to the gastroenterology clinic because she has been having 8 to 10 large-volume, watery stools daily for the past 6 weeks. The stools contain no blood or mucus. She reports no fever, no abdominal cramping, and no recent antibiotic use. Her primary care physician supervised a 48-hour fast two weeks ago, during which the diarrhea continued at the same frequency and volume. On examination, her abdomen is soft with mild diffuse tenderness and no rebound. Stool studies show sodium 110 mEq/L, potassium 20 mEq/L, no fecal leukocytes, and a calculated stool osmotic gap of 30 mOsm/kg. Serum electrolytes are within normal limits.
Which of the following best describes the mechanism of this patient's diarrhea and the significance of the stool osmotic gap?