MRCP2-1963
A 23-year-old teacher presented to the clinic with a complaint of chronic diarrhea for the past year. She reported that her stools were difficult to flush away and floated. She denied any weight loss and maintained her appetite. She had traveled to Blackpool eight months ago. Two months ago, a stool culture revealed Giardia lamblia, and she was treated with oral metronidazole for seven days, but her symptoms did not improve. She had no significant medical history and was not taking any regular medication. Her mother had scleroderma. On examination, she appeared thin but well, with no fever or lymphadenopathy. Her chest was clear, heart sounds were normal, and her abdominal examination, including a per rectal examination, was unremarkable. What would be the most appropriate investigation to perform next?