A 48 year old man presents with episodic watery diarrhea for five months, which was predominantly nocturnal. He had also lost 8 kg of weight during the preceding year.

His medical history is unremarkable, and he is not on any drugs. There is no significant history of travel.

His serum albumin is found to be 2.5 g/dL (normal: 3.5 to 5.5 g/dL).