A 48-year-old male with a history of hepatitis B presented with nephrotic syndrome 2 weeks ago. Physical exam: BP 165/98 mmHg, no hepatosplenomegaly, ascites (+). Laboratory tests: urine protein (++++), RBC 10-15/HPF, WBC 0-2/HPF, 24-hour urinary protein 4.0g, non-selective proteinuria, normal complement and renal function. No evidence of malignancy. The most likely diagnosis is: