A 32-year-old woman with a 10-year history of systemic lupus erythematosus (SLE) presents with sudden-onset gross haematuria, proteinuria (4.2 g/day), and hypertension (148/92 mmHg). Serum creatinine has risen from 0.9 to 2.1 mg/dL over 1 week. Complement C3 and C4 are low, and anti-dsDNA antibodies are elevated. Urinalysis shows RBC casts, WBC casts, and dysmorphic RBCs. What is the most appropriate immediate next step in management?