A 36-year-old woman complains of nausea, belching, liquid stool, and a pain in the epigastrium after meals. For the last 2 years the disease has been slowly progressing. Objectively, her skin is pale and dry, her tongue is coated, moist, and has imprints of the teeth on its edges. Abdominal palpation detects a diffuse pain in the epigastrium. What test will be the most informative in this case and should be conducted next ?