A 55-year-old man from Delhi undergoes total thyroidectomy for Graves' disease refractory to antithyroid drugs and beta-blockers. Intraoperatively, the surgeon identifies and carefully preserves all four parathyroid glands and the recurrent laryngeal nerves bilaterally. On postoperative day 2, the patient develops perioral paresthesias, positive Chvostek's sign, and a serum calcium of 7.2 mg/dL (normal 8.5–10.5). What is the most likely cause of hypocalcemia in this patient?