Medical mystery: an unusual cause of anaemia

An 82-year-old Indian man presented with a month's history of increasing lethargy, shortness of breath and 2 kg weight loss. No significant past illness was elicited, specifically no history of change in bowel habit, malaena, haematemesis, fever, abdominal or chest pain. At admission, he was on oral iron therapy only and was a non-smoker who rarely drank alcohol. On examination he was pale, haemodynamically stable, with normal cardiorespiratory, abdominal and digital rectal examinations.