Jonathan who is 57 years old is concerned about an enlarging mass in his neck. He first noticed a lump approximately six months ago and is worried now given it has not self-resolved. 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?