A 9 1/2 -year-old girl presents to your office with an approximately 1 1/4 -cm nodule in her neck, just to the left of the midline and below her cricoid cartilage. It is nontender and moves when she swallows. It has been enlarging over the last several months and was not seen by the pediatrician at her 9-year-old check-up. There is no family history of endocrine disorders. The most likely diagnosis is