A 54-year-old man presents to your office after noticing a lump in his neck that has been slowly growing in size. Three months ago, he started to have episodic diarrhea, with severity ranging from mild to severe. Several times a week, his face and neck suddenly turns red for a few minutes. His current medications include verapamil 120 mg three times a day and sildenafil 50 mg as needed. He reports that many in his family and relatives have cancers of thyroid, parathyroid and gastrointestinal system. During his physical examination, you note that he has a nontender nodule in the anterior neck as well as some cervical lymphadenopathy. You became concerned and ordered a blood test to check his thyroid hormone and TSH levels, a radioactive thyroid scan, and an FNA biopsy of both the lump in his neck and his enlarged cervical lymph nodes. Fine biopsy results showed neuroendocrine cells appearing as sheets of neoplastic cells with abundant, interspersed amyloid which stains Congo red. Mutational analysis shows positive for RET proto-oncogene. Which of the following is a useful tumor biomarker to diagnose this patient’s disorder?
