An elevated BUN to creatinine ratio greater than 20:1 most commonly suggests which type of azotemia?

• A. Postrenal obstruction
• B. Prerenal causes
• C. Intrinsic renal disease
• D. Urinary tract infection

Answer: (B)

When the BUN to creatinine ratio is markedly high, it points to decreased kidney perfusion rather than intrinsic kidney damage. In prerenal states, the kidneys detect reduced blood flow and respond by reabsorbing more water and urea in an effort to conserve volume. This preferential reabsorption raises BUN much more than creatinine, so the ratio goes above about 20:1.

In contrast, intrinsic renal disease damages the tubules themselves and impairs reabsorption, so BUN does not rise disproportionately to creatinine; the ratio tends to be closer to normal or only mildly elevated. Postrenal obstruction can elevate both values, but the ratio is not characteristically high in the same consistent way as in prerenal azotemia. Urinary tract infections don’t directly explain a high BUN:Cr ratio.

So, a BUN to creatinine ratio greater than 20:1 most commonly suggests prerenal azotemia due to reduced renal perfusion.