Acanthosis Nigricans — Velvety Hyperpigmented Neck

A 13-year-old obese boy from rural Maharashtra presents with his mother who is concerned about dark, velvety thickening on the back of his neck that has gradually worsened over the past 6 months. On examination, the structure marked **A** in the diagram shows symmetric brown-to-black velvety plaques with a papillomatous surface in the posterior neck and axillae. Which of the following best explains the pathophysiological mechanism underlying this clinical finding?