A 52-year-old man from Delhi presents with fatigue and dyspnea for 4 months. He denies any history of bleeding. On examination: pallor is noted. Blood investigations: Hb 8.5 g/dL, MCV 68 fL, MCH 20 pg, RBC 6.2 × 10¹²/L, WBC 7.5 × 10⁹/L, platelets 420 × 10⁹/L. Serum iron 28 μg/dL, TIBC 450 μg/dL, ferritin 6 ng/mL. Peripheral blood smear: microcytic hypochromic RBCs, occasional target cells, no schistocytes. Upper GI endoscopy and colonoscopy are normal. What is the most appropriate next investigation to identify the source of iron loss?