A 4-year-old girl presents with swelling of the legs and ankles. Physical examination reveals pitting edema of the lower extremities. Urinalysis show 2+ proteinuria. The urinary sediment contains no inflammatory cells or red blood cells. Serum levels of BUN and creatinine are normal. The patient recovers completely after a course of corticosteroids. For the patient, electron microscopy of a renal biopsy specimen prior to treatment would most likely demonstrate which of the following abnormalities?