A 16-year-old boy is found to have hypertension on routine evaluation. He has no symptoms of shortness of breath or chest discomfort, but occasionally on exertion notes that his legs get tired easily. He has no other past medical history.On physical examination, the blood pressure in his arms is 140/90 mm Hg (bilaterally). Measurement of the blood pressure in his legs is 20mm Hg lower than in the arms. The remaining physical examination is normal. Which of the following is the most likely diagnosis?