A 45-year-old man comes to the physician because of a 3-week history of swelling of his legs. He has history of IV drug use and has hepatitis B and C. His blood pressure is 170/95. Physical examination shows no abnormalities. Laboratory studies and a blood smear are shown:
Hemoglobin 10 g/dL
Platelets 200,000/μL
Serum urea nitrogen 20 mg/dL
Creatinine 2.2 mg/dL
AST 60 U/L (NR 5-40)
ALT 70 U/L (NR 7-56)
Urine blood 2+
Urine protein 3+
Which of the following is the next best step in diagnosis?
