A 31-year-old male patient with a history of Crohn’s disease presents to the emergency department with fever and weakness. He has four loose nonbloody bowel movements per day. Initial labs show white blood count of 1.4 × 10⁹/L, hemoglobin of 6.2 g/dL, and platelet count of 61,000. He reports his medications were recently changed to immunomodulators in order to wean off his steroids. What testing will be most useful in making a diagnosis?