A 48-year-old man presents with a complaint of non-bloody diarrhea and right lower quadrant pain with a palpable mass and tenderness. He states that this "flare-up" is one of the worst he has ever experienced. Radiographic examination reveals evidence of ulceration, stricture, and fistula development of the colon and small bowel. Which of the following drugs would be MOST useful for treating this patient?