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
All of the following are categorised as secondary lymphoid organs except - 
Select and explain. 
A. Lymph nodes, B. Spleen, C. Thymus, D. Subepithelial collections of lymphocytes
C: Thymus
Answer: C: Thymus 
Explanation: None