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?

A. Serum sodium levels: Desmopressin is used to treat diabetes insipidus by reducing urine output and increasing water retention. Monitoring serum sodium levels is critical because excessive water retention can lead to hyponatremia, which is a serious complication. B. Blood urea nitrogen (BUN): While BUN can be used to assess kidney function, it is not as directly impacted by desmopressin therapy in DI. Sodium levels are a more immediate concern for this client. C. Serum phosphate levels: Phosphate levels are not directly influenced by desmopressin or diabetes insipidus. Monitoring sodium levels is more crucial in this context. D. Serum calcium levels: Serum calcium levels are important in many conditions, but they are not the primary focus when a client is receiving desmopressin for diabetes insipidus. Monitoring sodium is more relevant.