This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Not a middle mediastinal structure ? 
Select and explain. 
A. Pulmonary trunk, B. Phrenic nerve, C. Thymus, D. Hea
C: Thymus
Answer: C: Thymus 
Explanation: Thymus