A 16-year-old male was admitted with sudden onset of severe edema. Blood pressure was normal, ascites sign (+), urinary protein (++++), red blood cells 0-2/HPF, 24-hour urinary protein quantification 6g, serum creatinine (Cr) 100μmol/L, blood C3 and CH50 normal, serum albumin 24g/L. After admission, prednisone 40mg daily was given orally. 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 after 6 weeks of treatment, with complete remission (CR) achieved by 8 weeks. The most likely clinical diagnosis is: