A 63-year-old man with poorly controlled type 2 diabetes presents with burning pain and numbness in both feet that has progressively worsened over the last two years. He describes the sensation as "walking on pins and needles." On examination, sensation to vibration and pinprick is reduced bilaterally in a stocking distribution, and ankle reflexes are absent. His pedal pulses are palpable. Which diagnosis best explains these findings? A. Lumbar radiculopathy B. Peripheral arterial disease C. Diabetic distal symmetric polyneuropathy D. Guillain-Barré syndrome E. Myasthenia gravis