There All Along

Question:  A 6 year old female is referred to you for evaluation of a neck mass.  She recently started gymnastics classes and noted swelling and pain in the left neck and shoulder when performing handstands.  On examination you find a soft, 6 cm mass just behind the sternocleidomastoid muscle.  Ultrasound reveals a fluid filled mass without a solid component.  Doppler does not reveal prominent supplying vasculature.  What is your diagnosis?  [Answer will be posted with next week's new question]

Answer to last week's question, "The Voice" (August 15, 2016): 

Recurrent respiratory papillomatosis.