A 28-year-old man is evaluated for a 2-month history of progressive lower extremity edema, weight loss, and fatigue. Medical history is significant for recreational use of inhaled cocaine; he denies injection drug use. He has no other known medical issues and takes no medications.
On physical examination, temperature is 37.2 °C (99.0 °F), blood pressure is 130/90 mm Hg, pulse rate is 90/min, and respiration rate is 20/min. BMI is 28. Temporal wasting is present. The lungs are clear. Cardiac examination is normal, and no pericardial rub is detected. There is no hepatosplenomegaly or evidence of ascites on abdominal examination. The lower extremities show edema to the knees bilaterally. Skin and joint examinations are normal. Mild asterixis is noted.
Albumin | 2.5 g/dL (25 g/L) |
Liver chemistry studies | Normal |
Blood urea nitrogen | 98 mg/dL (35 mmol/L) |
Creatinine | 6.8 mg/dL (601.1 µmol/L) |
Urinalysis | 1+ blood; 3+ protein; 5 erythrocytes/hpf; 0-2 leukocytes/hpf |
Urine protein-creatinine ratio | 3700 mg/g |
Kidney ultrasound shows mildly enlarged and echogenic kidneys without obstruction.
Kidney biopsy results are indicative of the collapsing variant of focal segmental glomerulosclerosis (FSGS).
Which of the following tests is most likely to establish the cause of this patient's FSGS?