This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
What is the main constituent of the plasma membrane? 
Select and explain. 
A. Carbohydrate, B. Lipid, C. Phospholipid, D. Protein
D: Protein
Answer: D: Protein 
Explanation: The approximate composition is proteins: 55 percent; phospholipids: 25 percent; cholesterol: 13 percent; other lipids: 4 percent; and carbohydrates: 3 percent. .