A 66-year-old woman was brought to the emergency department with a complaint of dyspnea and chest pain on exercise of left arm. She also had dizziness while turning her face to the left. She underwent coronary artery bypass graft surgery 4 years ago. On physical examination, her blood pressure was 115/67 mmHg in the left arm and 139/82 mmHg in the right arm. The findings of coronary angiography revealed retrograde flow from the left anterior descending artery to the left IMA. Aortography showed that the ostium of the left subclavian artery had a severe stenosis and that the left vertebral artery was visualized retrogradely. What could be the probable diagnosis?