Jonathan who is 57 years old has presented today as he is concerned about an enlarging mass in his neck. He first noticed a lump approximately six months ago but felt it is was likely due to ongoing dental issues. His wife is concerned that it might be something more sinister prompting his visit today. Jonathan is known to have hypertension, high cholesterol and hypothyroidism. He is a non-smoker and doesn’t drink alcohol.
Examination reveals a pulsatile mass in the posterior triangle that can be moved from side-to-side but not up and down.
What is the most likely diagnosis?