A 64-year-old man presents with fatigue and dyspnoea on exertion. His haemoglobin is 88 g/L, mean corpuscular volume 70 fL and ferritin 8 µg/L. He denies overt gastrointestinal bleeding but reports a 6-month history of decreased appetite and 5 kg weight loss. According to Canadian practice, what is the most appropriate next investigation?