Final answer:
In pre-renal acute renal failure, the BUN/Creatinine ratio increases due to factors that reduce blood flow to the kidneys, leading to higher reabsorption of urea and relative stability in creatinine levels.
Step-by-step explanation:
The BUN/Creatinine ratio in pre-renal acute renal failure typically increases. Pre-renal uremia, which is a type of uremia occurring before the kidneys are affected, can lead to a significant rise in the blood urea level, up to 300 mg/dl. This condition is often caused by factors external to the kidneys that reduce blood flow to the kidneys and impair kidney function, leading to an accumulation of nitrogen waste products in the blood, as kidneys are not able to filter waste efficiently due to the decreased flow.
When pre-renal factors such as dehydration, heart failure, or shock cause a decrease in renal perfusion, the kidneys are still functional. As a result, the reabsorption of urea by the renal tubules increases, leading to a higher BUN level relative to creatinine, which is filtered but not reabsorbed. Creatinine production remains relatively constant, so an elevated BUN/Creatinine ratio can be indicative of pre-renal causes of acute kidney injury.