Glomerular Diseases USMLE Step 1 Practice Question

A 62-year-old man with a 15-year history of type 2 diabetes mellitus presents to the clinic with progressive lower extremity edema and frothy urine over the past 3 months. He reports no recent infections or medication changes. Vital signs include BP 158/92 mmHg and HR 88/min. Laboratory studies show: serum creatinine 1.1 mg/dL, 24-hour urine protein 8.5 g/day, serum albumin 2.2 g/dL, total cholesterol 298 mg/dL, serum glucose 187 mg/dL, and normal complement levels (C3, C4). Urine microscopy shows no RBCs or casts. Renal ultrasound demonstrates normal-sized kidneys. A renal biopsy is performed, and light microscopy reveals diffuse glomerular basement membrane thickening. Immunofluorescence shows granular IgG and C3 deposits. Electron microscopy demonstrates thickened GBM with subepithelial immune complexes creating a characteristic "spike-and-dome" appearance. Which of the following is the most likely diagnosis?