A 13-year-old otherwise healthy girl presented to the emergency department with a 3-week history of progressive swelling of the right side of her neck and underneath the right side of her tongue. The swelling had been causing discomfort while eating. She denied any fever, vomiting, cough, changes in voice, difficulty breathing, oral injury, or oral surgery. Her immunization status was up to date. On her physical examination, vital signs were within normal limits. A dome-shaped, bluish, translucent, firm, movable, and nontender swelling was present on the right-sided floor of the mouth. Also noted was an extension of the swelling to the right side of the girl’s neck, involving the submental and submandibular region, with no overlying redness or warmness of skin. The rest of her physical examinations were unremarkable. A computed tomography (CT) scan of the neck with contrast showed a cystic lesion measuring approximately 4.5 cm in the largest dimension, with no significant peripheral enhancement, extending from the right sublingual space to the anterior submandibular space. Anteriorly, the lesion also crossed the midline to the left sublingual space. How would you diagnose this lesion?