MSRA-0663

A 14-year-old girl presents to a gastroenterology clinic with a history of diarrhoea, bloating, and severe abdominal pain for the past 8 months. Her full blood count results are as follows:
Hb 118 g/L Male: (135-180)
Female: (115 – 160)
Platelets 190 * 109/L (150 – 400)
WBC 7.5 * 109/L (4.0 – 11.0)

What would be the most appropriate next investigation?