Participants will be randomized to receive either intravenous (IV) infliximab monotherapy (Top-Hold approach) starting at a dose of 5 mg/kg at Weeks 0, 2, and 6 and thereafter every 8 weeks in Level 1, or classical Step-Up treatment starting with oral prednisolone (40 mg/day for at least 3 days and at most 2 weeks followed by 1 mg/kg/day for a minimum of 7 days and up to 2 weeks in the case the participant does not show an improvement in clinical symptoms under 40 mg/day treatment) + oral 5-aminosalicylic acid (5-ASA) at a dose of 2 g/day in Level 1.