A 45-year-old woman presents to her general practitioner with a 6-month history of intermittent diarrhea, abdominal pain, and weight loss. She reports that her symptoms are exacerbated by stress and certain foods, particularly those containing gluten. She has a family history of autoimmune thyroid disease. On examination, she appears pale and has a slightly distended abdomen with mild tenderness in the lower quadrants. Laboratory tests reveal microcytic anemia and elevated tissue transglutaminase antibodies. Which of the following is the most appropriate next step in the management of this patient?