Kidney biopsy for suspected MGRS in a diabetic patient
A 68-year-old woman with long-standing type 2 diabetes complicated by neuropathy and hypertension is evaluated for proteinuria and increased serum creatinine. Her medications include glipizide, valsartan, amlodipine, atorvastatin, and aspirin. She reports no changes to her medication regimen and does not use over-the-counter medications, herbal supplements, or NSAIDs. She feels well and reports no problems with urination, changes in weight, fever, rash, arthralgias, or cardiopulmonary symptoms. BP is 142/60 mm Hg, heart rate is 75/min, and body mass index is 29 kg/m². On examination, she has retinal cotton wool spots and 1+ edema of her lower extremities bilaterally. Laboratory data (6 months ago, current): WBC count 8000/µL, 7300 (reference 4000–11,000); hemoglobin 12.6 g/dL, 12.4 (12–16); platelet count 230,000/µL, 263,000 (150,000–450,000); potassium 4.5 mEq/L, 4.7 (3.5–5.0); creatinine 1.1 mg/dL, 1.9 (0.5–1.1); calcium 9.0 mg/dL, 9.2 (8.6–10.2); albumin 3.9 g/dL, 3.8 (3.5–5.5); total protein 7.1 g/dL, 6.8 (5.5–9.0); hemoglobin A1C 7.2%, 7.3 (4.0–5.6); urine protein-to-creatinine ratio 0.8 mg/mg, 2.5 (<0.2); free kappa light chain 30 mg/L (3.3–19.4); free lambda light chain 7 mg/L (5.7–26.3); free kappa-to-free lambda light chain ratio 4.28 (0.26–1.65). Urinalysis demonstrates 3+ protein but is otherwise normal. Urine microscopy demonstrates occasional granular casts. Serum protein electrophoresis (SPEP) and immunofixation are normal. Urine protein electrophoresis (UPEP) and immunofixation reveal 2.5 g protein/24 h consisting of 90% albumin with no Bence-Jones proteins. Kidney ultrasound demonstrates bilateral 11-cm kidneys with mildly increased echogenicity. The postvoid bladder volume is 25 mL. Which ONE of the following is the MOST appropriate next step?