A 25-year-old man has a 5-year history of celiac sprue. Several days after a mild upper respiratory infection, he begins passing dark red-brown urine. The dark urine persists for the next 3days and then becomes clear and yellow, only to become red-brown again l month later. There are no remarkable findings on physical examination. Urinalysis shows a pH of 6.5, specific gravity 1.018, 3+hematuria, l+ proteinuria, and no glucose or ketones. Microscopic examination of the urine shows RBCs and no WBCs, casts, or crystals. A 24-hour urine protein level is 200mg. A renal biopsy specimen from the glomeruli of this patient is most likely to show which of the following alterations?