This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
Essential difference between serum and plasma is in content of 
Select and explain. 
A. Protein, B. Sugar, C. Fat, D. RBC
A: Protein
Answer: A: Protein 
Explanation: Ans: a (Protein) Ref: Ganong 22nd edp.543Serum = Plasma - (fibrinogen, clotting factors II, V, VIII).Serum has higher serotonin content because of breakdown of platelets during clotting.Normal plasma volume is about 5% of body weight, -3500 ml in 70 kg man.