A markedly elevated BUN/creatinine ratio (>20:1) is most suggestive of which condition?

• A. Prerenal azotemia due to reduced renal perfusion.
• B. Intrinsic renal failure.
• C. Postrenal obstruction.
• D. Hepatorenal syndrome.

Answer: (A)

The main idea here is how the BUN to creatinine ratio helps distinguish where the kidney problem is coming from. A markedly elevated ratio greater than 20:1 points to prerenal azotemia, usually from reduced renal perfusion. When kidney blood flow falls, the kidneys conserve water and reabsorb more urea in the proximal tubule, so BUN rises disproportionately while creatinine is filtered and not reabsorbed. That makes the BUN/creatinine ratio go up.

In intrinsic renal failure, the damage is within the kidney itself and tubules lose their ability to reabsorb and excrete substances properly, so the ratio tends to be normal or only mildly elevated. Postrenal obstruction can cause azotemia as well, but the ratio is variable and not as consistently elevated as in prerenal states. Hepatorenal syndrome involves reduced kidney function in the setting of liver disease, and while kidney function is impaired, the ratio is not the defining clue.