A 28-year-old man from Delhi presents with acute-onset severe abdominal pain, profuse watery diarrhea (15–20 stools/day), and fever 3 days after undergoing elective appendicectomy under general anesthesia. On examination, he is dehydrated and has diffuse abdominal tenderness. Stool is bloody. Flexible sigmoidoscopy shows pseudomembranes on the colonic mucosa. What is the most appropriate immediate next step?