A 50-year-old male patient was admitted with 'rash on both lower limbs, abdominal pain, and arthralgia for 1 week, and edema for 3 days'. Physical examination: BP 160/90 mmHg. Renal pathology showed: immunofluorescence IgG(-), IgA(+++), IgM(++), C3(-), C1q(-), Alb(-), with mass-like and granular deposits in the mesangial area. Light microscopy revealed 22 glomeruli, 2 cellular crescents, 2 global sclerosis, and mild diffuse proliferation of mesangial cells and matrix with mesangial eosinophilic deposits. There was multifocal mononuclear and lymphocyte infiltration in the renal interstitium. If urinalysis shows proteinuria (+++), RBCs 20–25/HPF, and 24-hour urinary protein quantification of 2.58 g, which treatment should be administered?