Melissa Eggers, a 65-year-old retired cook, has a history of type 1 diabetes mellitus. She presents to the clinic, where you practice nursing, because of sudden onset of dizziness. Ms. Eggers is morbidly obese and uses a walker to ambulate. Vital signs are pulse 98 beats/min, respirations 18 breaths/min, and blood pressure 170/90 mm Hg. Her dorsalis pedis pulse is weak bilaterally; she has bilateral carotid bruits. Neurologically, she is intact. What might be a cause of Ms. Eggers’s dizziness?