A 58-year-old woman from the Himalayan region presents with a 5-year history of progressive neck swelling and recent onset dyspnea on exertion and positional stridor. Examination reveals facial plethora and distended neck veins when both arms are elevated above the head. Thyroid function tests show TSH 1.2 mIU/L, free T4 12 pmol/L (normal). CT neck and thorax reveals the structure marked **A** in the diagram — asymmetric multinodular thyroid enlargement with retrosternal extension compressing the trachea. Which of the following best explains the pathogenesis of the multinodular pattern seen in structure **A**?