Josh is a five year old boy who has been brought to see you by his mother who is concerned about a neck lump. She first noticed it a few months ago but thought it would self-resolve. Josh is otherwise fit and well, up to date with immunisations and does not have any significant family history. Examination reveals a small swelling located at the lateral edge of the thyroid cartilage. It moves upwards when his tongue is protruded.
What is the most likely diagnosis?