A 55-year-old woman with a 10-year history of diabetes mellitus and hypertension presents with a 3-day history of progressive pain and coldness in her right foot. She reports no prior claudication. On examination, the right foot is mottled and cyanotic, with absent pulses below the popliteal artery. Sensory examination shows diminished sensation in the foot and lower leg. Motor examination reveals mild weakness of foot dorsiflexion. Capillary refill is markedly delayed. What is the most likely underlying mechanism of limb ischemia in this patient?