A 23-year-old patient complains of a dull ache, sensation of heaviness and distention in the epigastrium immediately after meals, foul-smelling eructation; dry mouth, empty stomach nausea, diarrhea. Objectively: the skin is pale, the patient is of thin build. Abdomen is soft on palpation, there is epigastric pain. The liver does not extend beyond the costal arch. In blood: Hb - 110 g/l, RBCs - 3,4 × 1012/l, WBC count is normal. ESR - 16 mm/h. What is the most informative study that will allow make a diagnosis?