12. A 38-year-old woman who had been diagnosed as membranous nephropathy with heavy proteinuria (4.5 g/day). One day she presented with flank pain, tenderness, hematuria, and rapid decline in renal function. Which is the most likely diagnosis ?
(A) Renal stone with urinary tract infection
(B) Renal vein thrombosis
(C) Renal artery stenosis
(D) Malignant hypertension
(E) Thrombotic microangiopathy