A nurse is providing care for a 71-year-old client who has diabetes insipidus (DI) and is receiving treatment with desmopressin. Which of the following lab values is most important for the nurse to monitor?

Desmopressin acts as a potent V2 receptor agonist that maximizes renal water reabsorption, directly reversing free water wasting. In geriatric patients, age-related declines in glomerular filtration and altered thirst mechanisms increase the risk of water intoxication. Rapid fluid retention can precipitate severe dilutional hyponatremia, leading to acute cerebral edema and neurological collapse. A. Blood urea nitrogen (BUN): While urea levels provide insights into renal perfusion and hydration status, they fluctuate with dietary protein intake and hepatic function. It does not reflect acute water-retention complications as sensitively as direct electrolyte measurements. It is secondary to monitoring ion concentrations. B. Serum phosphate levels: Phosphate homeostasis is regulated primarily by parathyroid hormone and proximal renal tubular transport, running independently of vasopressin-mediated water channels. Desmopressin administration does not alter phosphate excretion or cause critical shifts. It is not a priority lab for this medication. C. Serum sodium levels: Upregulated water reabsorption expands vascular volume, creating a high risk for rapid drop in sodium concentration, especially in older adults. Monitoring for hyponatremia is mandatory to prevent drug-induced encephalopathy, seizures, or altered mental status. It is the most critical lab parameter to track. D. Serum calcium levels: Calcium concentrations depend on vitamin D activity, bone turnover, and parathyroid regulation rather than isolated free water clearance in the collecting ducts. Fluctuations in water volume do not induce acute, hazardous shifts in ionized calcium. It lacks relevance for monitoring desmopressin safety.