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: Physiology
ABO antigens are not present in ? 
Select and explain. 
A. Saliva, B. CSF, C. Blood, D. Tear
B: CSF
Answer: B: CSF 
Explanation: Ans. is 'b' i.e., CSF"Soluble ABO antigens are detected in secretions and all body fluids except cerebrospinal fluid".