A 28-year-old woman known to have ulcerative colitis (UC) presents with a 1-week history of increased stool frequency (8 bloody stools per day), abdominal cramps, and urgency. She feels generally unwell. Her CRP is 60 mg/L and ESR is 55 mm/hr. According to NICE guidelines for managing a moderate flare of UC, what is the most appropriate initial step?