Q. A 68-year-old man presents to the clinic with a 6-month history of lower extremity pain. He describes the pain as a cramping sensation localized to his calves that intensifies when he walks and promptly subsides upon sitting or resting. He denies experiencing any associated chest pain, palpitations, or shortness of breath. His medical history is notable for hypertension, hyperlipidemia, and a 30-pack-year smoking history. On physical examination, his pulses in the dorsalis pedis and posterior tibial arteries are notably diminished when compared to his radial pulses. His feet are warm to the touch, with intact sensation and no evident ulcers or signs of gangrene. His laboratory investigations reveal a complete blood count within normal limits. However, his lipid profile shows an elevated LDL cholesterol at 160 mg/dL and a slightly reduced HDL cholesterol at 35 mg/dL. ECG shows a normal sinus rhythm without any signs of ischemic changes. A treadmill exercise test was performed, during which he reported the onset of his typical calf pain after just 2 minutes of walking. Which of the following tests would be the most appropriate initial step in evaluating this patient's condition?