Review Question - QID 102606

A 24-year-old woman presents to her primary care physician with a longstanding history of diarrhea. She reports recurrent, foul-smelling, loose stools and a 35 lb weight loss over the past 3 years. She also states that two months ago, she developed an "itchy, bumpy" rash on her elbows and forearms which has since resolved. She denies recent camping trips or travel outside of the country. On physical exam she appears thin, her conjunctiva and skin appear pale, and her abdomen is mildly distended. Which of the following tests would confirm this patient's diagnosis?