Question

During morning rounds, the nephrology service evaluates a newly admitted patient presenting with severe anasarca and new-onset renal dysfunction. The patient, a $58$-year-old individual with a history of hypertension, reports noticing persistently frothy urine for the past $12$ weeks. Vital signs show a blood pressure of $152/94 \text{ mmHg}$. Laboratory studies demonstrate a serum creatinine of $1.8 \text{ mg/dL}$ and a serum albumin level of $2.5 \text{ g/dL}$. Urinalysis reveals $4+$ proteinuria with microscopic hematuria, and a $24$-hour urine collection confirms the presence of $5.2 \text{ g}$ of protein. A percutaneous renal biopsy is obtained. Light microscopy reveals prominent mesangial expansion and capillary wall thickening. Congo red staining demonstrates no evidence of apple-green birefringence under polarized light. Electron microscopy reveals dense deposits composed of randomly oriented, non-branching fibrils measuring $18 \text{ nm}$ to $22 \text{ nm}$ in diameter located within the mesangium and glomerular basement membranes. Immunohistochemical staining of the biopsy tissue shows strong positivity for DNAJB9. Based on these histopathologic and molecular findings, which of the following is the most likely diagnosis?