A 58-year-old woman from Mumbai with a 15-year history of type 2 diabetes mellitus presents with progressive darkening and thickening of skin in the neck, axillae, and inframammary folds over the past 2 years. Skin biopsy shows acanthosis with papillomatosis and basal hyperpigmentation. Physical examination reveals no lymphadenopathy, and her weight is stable. Fasting glucose is 180 mg/dL on current metformin and glipizide therapy. Chest X-ray and abdominal ultrasound are unremarkable. What is the most likely etiology of her acanthosis nigricans?