Multiple Endocrine Neoplasia Type 2B

A 14-year-old boy presents with a neck mass, mucosal neuromas on the lips and tongue, marfanoid habitus with long limbs and high-arched palate, and episodic hypertension with palpitations. His father died of medullary thyroid carcinoma at age 28. Serum calcitonin is markedly elevated at 1200 pg/mL, and 24-hour urinary metanephrines are 4× the upper limit of normal. MRI confirms bilateral pheochromocytoma. Genetic testing reveals the mutation shown in the karyotype marked **A** in the diagram. Which of the following is the MOST APPROPRIATE immediate management step for this patient?