A 28-year-old woman with ulcerative colitis confined to the rectum and sigmoid colon presents with 6–8 bloody stools daily, tenesmus, and mild systemic symptoms. Flexible sigmoidoscopy shows continuous mucosal erythema, friability, and ulceration limited to the rectosigmoid. Laboratory studies are unremarkable except for mild anemia (Hb 10.2 g/dL). What is the drug of choice for induction of remission in this patient?