A 63-year-old man complains of weakness without an obvious cause, fatigability, loss of appetite, and a feeling of disgust towards meat products. Two weeks ago he had a gastric bleeding. Objectively, t° – 37.5°C, respiratory rate – 20/min., pulse – 96/min., blood pressure – 110/75 mm Hg. Epigastric palpation detects pain and muscle tension. In the blood: Hb – 82 g/L, ESR – 35 mm/hour. What study will be the most useful for making a diagnosis?