A 6-year-old girl has become increasingly lethargic over the past 2 weeks. On examination, she has puffiness around the eyes. Her temperature is 36.9degC, and her blood pressure is 100/60 mm Hg. Laboratory findings show serum creatinine, 0.7 mg/ dL; urea nitrogen, 12 mg/dL; and cholesterol, 217 mg/dL. Urinalysis shows pH, 6.5; specific gravity, 1.011; 4+ proteinuria; lipiduria; and no blood or glucose. The 24-hour urine protein level is 3.8 g. The child's condition improves after glucocorticoid therapy. Which of the following findings by electron microscopy is most likely to characterize this disease process?