Final answer:
The ratio of pre-renal urine osmolality to plasma osmolality can vary based on hydration status but is generally higher than plasma osmolality because urine contains solutes excreted by the kidneys.
Step-by-step explanation:
The ratio of pre-renal urine osmolality to plasma osmolality is an important measure of kidney function, particularly in determining the kidney's ability to concentrate or dilute urine. A healthy kidney will adapt the urine osmolality according to the body's state of hydration. When the body is dehydrated, with restricted fluid intake, the urine osmolality can rise above 800 mOsm/kg reflecting a concentrated urine. Conversely, with increased fluid intake, a well-functioning kidney can produce urine with an osmolality as low as 50-100 mOsm/kg, indicating diluted urine. In cases where fluid balance and kidney function are normal, the urine osmolality can vary but will commonly be higher than plasma osmolality due to the solutes excreted.