A 15-year-old male presents with sudden-onset generalized edema for half a month. Examination: BP 120/70 mmHg, pitting edema of eyelids and lower limbs, urinary protein (+++), BLD (-), 24-hour urinary protein quantification 6.8g, serum albumin 20g/L, BUN 10.7mmol/L, Cr 125μmol/L. The first-line treatment is: