You are answering a USMLE-style medical question. Question: A 6-year-old boy presents with fever, malaise, and intense pain in the anterior neck. His vital signs include: body temperature 39.0°C (102.2°F), heart rate 120/min, and respiratory rate 18/min and regular. On physical examination, there is erythema, tenderness and enlargement of the thyroid gland that is worse on the left. Pain is worsened during neck hyperextension and relieved during neck flexion. Thyroid function tests are within normal limits. An ultrasound of the thyroid gland reveals a unifocal perithyroidal hypoechoic space. Which of the following is the most likely mechanism underlying this patient’s condition? Options: A) Postviral inflammatory process B) Pyriform sinus fistula C) Antithyroid peroxidase (TPO) D) Autoantibodies to the thyrotropin receptor (TRAb) Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.