A 22-year-old woman presents with recurrent episodes of watery diarrhoea, abdominal cramping, and bloating, often worse after eating certain foods, particularly bread and pasta. She has also noticed some weight loss and fatigue over the past year. Blood tests show a microcytic anaemia (Hb 105 g/L, MCV 78 fL). What investigation is most appropriate to screen for the likely underlying condition?