NBME Form 26 Practice Test

Mannitol is an osmotic diuretic that causes an increase in plasma osmolality when infused. This is due to mannitol's ability to draw water into the vascular space from the interstitial space, thereby concentrating the plasma and increasing its osmolality.

In response to this rise in osmolality, the body works to restore homeostasis. One of the physiological responses is the secretion of antidiuretic hormone (ADH), also known as vasopressin. Increased plasma osmolality stimulates osmoreceptors in the hypothalamus, leading to an increased release of ADH from the posterior pituitary.

ADH acts primarily on the kidneys to promote water reabsorption, thereby helping to decrease plasma osmolality over time as more water is reabsorbed into circulation. However, immediately following the infusion of mannitol, both the increase in plasma osmolality and the consequential increase in ADH are expected due to the osmotic effect of the mannitol in the circulation.

This understanding clarifies that the expected changes in both plasma osmolality and ADH concentration following mannitol infusion are an increase in plasma osmolality and an increase in plasma ADH.