This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, Subject name: Pathology
ABO antigens are not found in - 
Select and explain. 
A. CSF, B. Plasma, C. Saliva, D. Semen
A: CSF
Answer: A: CSF 
Explanation: option 1 is correct. REFERENCE; WINTROBE CLINICAL HAEMATOLOGY. Volume 1 page 636. Although the ABO blood group antigens are regarded as RBC antigens, they are actually present in wide variety of human tissues especially on most epithelial and endothelial cells. Blood cells such as T cellls, B cells and platelets also have ABO antigens. It is absent in CSF.