A 32-year-old woman with systemic lupus erythematosus (SLE) on mycophenolate mofetil (MMF) 2 g/day presents with severe diarrhea, abdominal cramping, and weight loss over 2 weeks. Laboratory investigations show hemoglobin 9.2 g/dL, WBC 2.1 × 10⁹/L, and platelets 85 × 10⁹/L. Stool culture and C. difficile toxin are negative. What is the most appropriate next step in management?