A 58-year-old man with type 2 diabetes mellitus for 12 years presents to the outpatient clinic with a painless ulcer on the sole of his foot over the metatarsal head region. On examination, the ulcer is well-demarcated with a punched-out appearance (marked **A** in the diagram), surrounded by a thick hyperkeratotic callus. His dorsalis pedis and posterior tibial pulses are palpable bilaterally, and the foot is warm. Which of the following is the PRIMARY pathophysiological mechanism responsible for the development of the ulcer marked **A**?