Review Question - QID 219834

You receive an inpatient consult to evaluate worsening hand swelling and discoloration over the past six hours. The patient is a 74-year-old male with a history of heart failure and venous insufficiency, who presented five days prior with a severe traumatic brain injury following a motor vehicle collision. He is currently intubated in the neurological ICU, where he is receiving IV phenytoin for the management of post-traumatic seizures. On examination, you note violaceous discoloration and pitting edema of his right hand, as shown in Figure 1. His right upper extremity proximal to the wrist and the remainder of his extremities are without abnormalities. Radiographs of the hand demonstrate soft tissue swelling without any osseous injury. White blood cell (WBC) count is 10,500/mm^3 and CRP is 8.2 mg/dL. You recommend discontinuing his ipsilateral hand I.V., strict extremity elevation, and serial monitoring, and his symptoms gradually improve over the following week without further intervention. Which of the following describes the most likely mechanism underlying this patient’s presentation?