A 38-year-old man with lupus nephritis (Class IV) is started on mycophenolate mofetil (MMF) 1 g twice daily as part of his induction therapy. At 3 weeks, he develops severe diarrhea (6–8 stools/day), abdominal cramping, and a white blood cell count of 2.8 × 10⁹/L (previously 7.2 × 10⁹/L). Hemoglobin is 10.2 g/dL. What is the most appropriate next step in management?