Final answer:
Urine osmolality is the most important assessment data to obtain for ongoing evaluation of Donna's fluid volume status.
Step-by-step explanation:
For ongoing evaluation of fluid volume status, the most important assessment data to obtain is urine osmolality. Urine osmolality is a measurement of the number of dissolved particles in urine and provides information about the ability of the kidneys to concentrate or dilute urine. By measuring urine osmolality, healthcare providers can assess the fluid balance in a patient and determine if there are any abnormalities in fluid volume status.