What effect does increased antidiuretic hormone have on serum sodium levels?

Increased levels of antidiuretic hormone (ADH), also known as vasopressin, lead to fluid retention in the body. ADH acts primarily on the kidneys, promoting the reabsorption of water back into the bloodstream, which decreases urine output. When the body retains more water without a corresponding increase in sodium intake or retention, it can lead to dilutional hyponatremia, meaning that the serum sodium levels become low due to the higher water content in the blood.

This mechanism helps understand why fluid retention occurs in the presence of increased ADH. As water is reabsorbed, the serum becomes more dilute, thus reducing the concentration of sodium in the blood. Therefore, with increased ADH, the most notable effect is fluid retention alongside low serum sodium levels, which aligns perfectly with the correct answer chosen.