Raynaud Phenomenon

A 24-year-old woman presents with a 3-year history of recurrent episodes of color change in her fingers triggered by cold exposure and emotional stress. Episodes last 15–30 minutes. During an acute episode, the structure marked **A** (well-demarcated digital pallor) is observed bilaterally at the MCP joints, followed by cyanosis and then reactive erythema on rewarming. She reports numbness and tingling during episodes. Examination between episodes shows normal hands with normal pulses and negative nailfold capillaroscopy. ANA and ESR are normal. Which of the following best explains the pathophysiology of the finding marked **A** in this patient?