MRCP-0476

A 43-year-old man is reviewed in the gastroenterology clinic. He has had troublesome dyspepsia for the past six months which has not settled with proton pump inhibitor (PPI) therapy. During the review of his systems he also reports passing 6-7 watery stools per day. An OGD 3 weeks ago showed gastric erosions and ulcers. Which one of the following investigations is most likely to be diagnostic?