A 58-year-old man with type 2 diabetes mellitus (HbA1c 9.2%) presents with a painless ulcer on the plantar surface of his foot over the metatarsal head (marked **A** in the diagram). He reports no history of trauma, and examination reveals diminished protective sensation to 10-g monofilament testing. The ulcer is superficial with surrounding hyperkeratotic callus. Which of the following best explains the pathogenesis of the ulcer formation at this pressure point?