Can the Past Predict the Future?

Question:    A 23 year old gentleman is referred to you for evaluation of a newly diagnosed right neck mass.  He is otherwise healthy.  He reports no prior surgery, but does have a history of penetrating neck trauma the year prior which was treated conservatively.  On palpation you note a mass in level II on the right.  This is pulsatile.  There are no overlying neck masses.  CT with contrast demonstrates a cystic neck mass.  What is the most likely diagnosis?  [Answer will be posted with next week's new question]

Answer to last week's question, What's on the Inside Matters Most (November 16, 2015): 

You should obtain a PTH level from the aspirate - indicative of a parathyroid cyst. In addition a thyroglobulin level can be obtained to determine if the origin is thyroid (for example, papillary thyroid carcinoma).