Final answer:
Prerenal failure is characterized by a BUN:creatinine ratio of 20:1 or higher, distinguishing it from acute renal failure due to intrinsic kidney disease, which would typically show a different ratio. The correct option is A.
Step-by-step explanation:
One of the key characteristics that differentiates prerenal failure from acute renal failure caused by renal disease is a BUN:creatinine ratio of 20:1 or higher.
This reflects a pre-renal cause of kidney impairment such as hypovolemia due to dehydration or shock, which leads to reduced blood flow to the kidneys without intrinsic kidney damage.
Conditions that cause prerenal failure do not usually result in an excess loss of sodium in the urine, which is why the BUN:creatinine ratio is employed as a distinguishing factor.