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
Which of the following does not describe a documented mechanism of tumor-mediated immune evasion? 
Select and explain. 
A. Down regulation of class I MHC, B. Secretion of TGF-b, C. Secretion of decoy molecules, D. Antigenic variation
D: Antigenic variation
Answer: D: Antigenic variation 
Explanation: Tumor cells have evolved mechanisms to escape and evade immune recognition. These mechanisms include various molecular genetic processes of eliminating MHC I from the tumor cell surface. Mutations inhibit efficient antigen processing and presentation. Tumor cells also block immune cells by secretion of suppressor proteins, most commonly TGF-b1, and other cytokine like "decoy" molecules to confuse the immune system. Antigenic variation is an immune escape mechanism ascribed to pathogenic microbes such as viruses for example.