A 52-year-old woman from Delhi presents with progressive weakness, bone pain, and recurrent kidney stones over 6 months. She reports nausea and constipation. Laboratory investigations show: serum calcium 11.2 mg/dL (normal 8.5–10.2), serum phosphate 2.8 mg/dL (normal 2.5–4.5), serum creatinine 1.4 mg/dL, 24-hour urinary calcium 380 mg/day (normal <250), and intact PTH 185 pg/mL (normal 10–65). Abdominal ultrasound reveals a 1.2 cm hypoechoic lesion in the lower pole of the right parathyroid gland. What is the primary mechanism responsible for hypercalcemia in this patient?