A 30-year-old woman presents with repeated episodes of watery diarrhea for almost one and a half years, in association with flatulence, and abdominal discomfort following meals. Further questioning reveals a weight loss of 10 kg during this time period.

Her medical and family histories are unremarkable, while she neither smokes nor drinks. There is no history of recent travel, or of drinking unfiltered water or unpasteurized milk.

A sample of stool is found to be frothy and foul smelling; they float when mixed with water. Microscopy shows no evidence of amoebae, ova, or cysts, and the stools test negative for blood. The fecal osmolar gap is 95 mOsm/kg.