Q. A 5-year-old boy presents to the pediatrician with a two-month history of painless swelling in the right side of his neck. He also reports a history of constipation and frequent diarrhea. His mother reports that he has a family history of thyroid cancer, which was diagnosed in her younger brother when he was 8 years old. The patient's vital signs on presentation are: blood pressure 130/90 mm Hg, pulse rate 170 beats/minute, and respiratory rate 20 breaths/minute. The physical examination reveals a palpable mass in the right side of his neck and high arched palate and long limbs. Further evaluation reveals elevated levels of calcitonin and carcinoembryonic antigen in the patient's blood and an ultrasound of the neck shows a solid mass in the right lobe of his thyroid. The patient undergoes a total thyroidectomy, and the pathology report confirms the diagnosis. Genetic testing reveals a RET proto-oncogene mutation. Further evaluation reveals bilateral pheochromocytomas, and the patient undergoes adrenalectomy. What is the most likely diagnosis for this patient?