A 19-year-old young man complains of a fistula on the neck anterior surface, which periodically reappears at the same place. Objectively: at the neck midline between the hyoid bone and thyroid cartilage there is a fistula; the skin of the affected area is scarred, drawn-in, and macerated. In the surrounding tissues a dense band extending from the fistula opening to hyoid bone can be palpated. A doctor has made a provisional diagnosis of thyroglossal fistula. Specify the additional method of investigation: