This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fluid & Electrolyte, Subject name: Medicine
Which one of the following is the major determinant of plasma osmolality - 
Select and explain. 
A. Serum sodium, B. Serum potassium, C. Blood glucose, D. Blood urea nitrogen
A: Serum sodium
Answer: A: Serum sodium 
Explanation: Ans. is 'a' i.e., Serum sodium o Plasma osmolality = 2 (sodium + potassium)+BUN2.8+Blood sugar18