This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
All are peripheral lymphoid organs except - 
Select and explain. 
A. Lymph nodes, B. Spleen, C. Mucosa associated lymphoid tissue, D. Thymus
D: Thymus
Answer: D: Thymus 
Explanation: Thymus & bone marrow are primary/central lymphoid organ, the spleen & lymph nodes are the secondary/peripheral lymphoid organ Reff: Ananthanarayanan & Paniker's textbook of microbiology 9th edition pg: 128