A 36-year-old male presents with generalized edema, urinary protein 8.6g/d, red blood cells (RBC) 5-10/HP in urine, fatty casts, and plasma albumin (Alb) 18g/L. Despite treatment with prednisone 60mg daily and dipyridamole 300mg (divided into 3 doses) for 8 weeks, there is no improvement. Which of the following measures is most appropriate?