A 12-year-old girl complains of headaches and blurred vision. She has a history of high blood pressure but is not currently taking medication. Her blood pressure is 160/95 mm Hg and pulse is 95 per minute. Funduscopic examination reveals small retinal microaneurysms and cotton-like zones of retinal edema and necrosis. She is hospitalized for further evaluation. Renal arteriography shows segmental stenoses forming multiple ridges that project into the lumen. What is the most likely cause of secondary hypertension in this young patient?