A 48-year-old woman from Delhi presents with progressive darkening and thickening of skin over the neck and axillae for 6 months. She denies weight loss and reports regular menstrual cycles. Physical examination reveals velvety, hyperpigmented plaques with exaggerated skin markings in the neck, axillae, and inframammary folds. BMI is 32 kg/m². Fasting blood glucose is 118 mg/dL, and HbA1c is 6.8%. Pelvic ultrasound and mammography are normal. What is the most likely underlying etiology of her acanthosis nigricans?