A 50-year-old woman with AKI (creatinine 350 μmol/L) is found to have a markedly positive ANCA (p-ANCA/MPO) with active urinary sediment (dysmorphic red cells, red cell casts) and proteinuria. She has purpuric skin lesions on her legs. Renal biopsy shows pauci-immune crescentic glomerulonephritis. What is the diagnosis and first-line treatment?