This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Physiology
Which of the following has the greatest effect on plasma osmolality? 
Select and explain. 
A. Progesterone, B. Coisol, C. Vasopressin, D. Aldosterone
C: Vasopressin
Answer: C: Vasopressin 
Explanation: Among the options, vasopressin (also called ADH or anti-diuretic hormone) has the greatest effect on plasma osmolality. Two primary system are especially involved in regulating the concentration of sodium and osmolarity of extracellular fluid: The osmoreceptor-ADH system and The thirst mechanism Although aldosterone decreases the rate of sodium excretion by the kidneys, the concentration of sodium in the extracellular fluid rises only by a few mEqs. The reason for this that when sodium is reabsorbed by the tubules, simultaneous absorption of equivalent amounts of water occurs. So, aldosterone does not cause a significant change in the plasma osmolality. Ref: Guyton and Hall 13th edition Pgno: 231, 232