MSRA-1964

A 5-year-old girl comes to the GP with her father. He is concerned that she is not growing as well as her classmates. The father reports that the child experiences foul-smelling diarrhoea about 3-4 times a week and complains of abdominal pain.
During the examination, the girl has a distended belly and thin buttocks. She has fallen 3 centile lines and now falls on the 15th centile.
What would be the most suitable first test to perform?