Pre-renal azotemia is best suggested by which finding?

• A. Elevated BUN and creatinine due to decreased renal perfusion
• B. Elevated BUN and creatinine due to intrinsic kidney damage
• C. High urine sodium excretion
• D. Low serum creatinine

Answer: (A)

Pre-renal azotemia reflects reduced blood flow to the kidneys. When perfusion drops, the kidneys conserve volume and increase proximal tubule reabsorption of urea, so BUN rises disproportionately to creatinine. This yields an elevated BUN and creatinine with a BUN:creatinine ratio often greater than 20:1. The urine tends to be concentrated with low sodium loss, reflecting avid reabsorption to preserve volume. So the finding that best fits prerenal azotemia is elevated BUN and creatinine due to decreased renal perfusion.

In contrast, intrinsic kidney damage elevates BUN and creatinine through different mechanisms and typically shows higher urine sodium despite impaired reabsorption, and the urine is not as concentrated. A very low serum creatinine would not indicate azotemia.