Acanthosis Nigricans of Insulin Resistance

A 15-year-old boy with obesity (BMI 33 kg/m²) presents with a 2-year history of progressive discoloration on the back of his neck and axillae that does not wash off. On examination, the lesions marked **A** in the diagram show a characteristic velvety, palpably thickened surface with symmetric distribution across the posterior neck and axillae. Laboratory investigations reveal fasting glucose 108 mg/dL, HbA1c 6.0%, and fasting insulin 32 μIU/mL (HOMA-IR 8.6). There is no history of malignancy, rapid weight loss, or dysphagia. Which of the following best explains the morphologic hallmark of the lesions marked **A**?