A 30-year-old woman presents with a 6-week history of bloody diarrhoea, urgency, and tenesmus. She is opening her bowels 6-8 times per day. She is systemically well and afebrile. Flexible sigmoidoscopy reveals continuous inflammation extending from the rectum to the sigmoid colon, with loss of vascular pattern and friability. Biopsies confirm ulcerative colitis. What is the most appropriate first-line treatment to induce remission?