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
What is marker for cell membrane? 
Select and explain. 
A. 5' nucleotidase, B. LDH, C. G6PD, D. All
A: 5' nucleotidase
Answer: A: 5' nucleotidase 
Explanation: A i.e 5-Nucleotidase