Which of the following may cause an increase in serum osmolality levels?

Mannitol therapy may cause an increase in serum osmolality levels because mannitol is an osmotic diuretic that increases the osmolarity of the blood. It works by drawing water out of cells and into the extracellular space, which elevates serum osmolality. This process helps to reduce cerebral edema and is often used in critical care settings, particularly for patients with conditions involving increased intracranial pressure.

In contrast to mannitol therapy, other options pose reasons for serum osmolality levels not to rise. Hyponatremia typically indicates low sodium levels in the blood, leading to a decrease in serum osmolality. Overhydration can dilute serum electrolytes and also contribute to lower serum osmolality. Similarly, the syndrome of inappropriate antidiuretic hormone secretion (SIADH) causes excess retention of water, resulting in dilutional hyponatremia and a decrease in serum osmolality. Thus, mannitol therapy stands out as the correct choice for causing an increase in serum osmolality levels.