1. Prepared by: Sintawat Wangsiricharoen MD

An adult with a history of neurofibromatosis type (NF) presents to an outpatient clinic with headache, tremor, periodic palpitation, jitters, and diarrhea. They also reports recent unintentional weight loss. Vital signs show blood pressure 127/95 mmHg, pulse rate 130/min, and respiratory rate 15/min. On examination, the thyroid is not enlarged and lungs are clear to auscultation bilaterally. The heart examination is noted for tachycardia, but no abnormal heart sounds. The extremities are slightly shaking with no edema. Initial lab studies are ordered. White blood cell count is 9,450/cu mm. Hemoglobin is 12.1 g/dL. Comprehensive metabolic panel shows Na 137 mmol/L, K 4.0 mmol/L, Cl 103 mmol/L, CO2 22 mmol/L, BUN 8 mg/dL, Cr 0.7 mg/dL, and Ca 9.5 mg/dL. Thyroid function tests show TSH 0.64 IU/mL (0.5-4.5 IU/mL), free T4 1.1 ng/dL (0.8-1.8 ng/dL), and free T3 2.4 pg/mL (2.0-4.9 pg/mL).

Question: Which one of the following tests is most helpful in making a diagnosis in this patient?