An 18-year-old male presents with nephrotic syndrome. 24-hour urinary protein is 4.5g, urine disc electrophoresis shows mid-molecular proteinuria (selective proteinuria), marked edema, normal blood pressure (BP) and renal function. He had a similar episode in childhood which achieved complete remission (CR) after standard steroid therapy, with negative urinalysis until this recurrence. After 8 weeks of treatment, proteinuria shows only slight reduction with no other significant improvement. At this stage, the best approach is: