A 16-year-old male presented with sudden severe edema, normal blood pressure (BP), ascites sign (+), urinary protein (++++), 0-2 red blood cells (RBC)/HPF, 24-hour urinary protein quantification 6g, serum creatinine (Cr) 100μmol/L, normal blood C3 and CH50, serum albumin 24g/L. After admission, he was given oral prednisone 40mg daily. Renal biopsy at 2 weeks showed mild mesangial proliferation, vacuolar degeneration of tubular epithelial cells, negative immunofluorescence, and foot process effacement on electron microscopy. Proteinuria significantly decreased at 6 weeks, with complete remission (CR) achieved by 8 weeks. The pathological diagnosis in this case is: