A patient with dehydration shows elevated BUN with a relatively normal creatinine. Which statement best describes the likely cause?

• A. Prerenal azotemia due to dehydration
• B. Intrinsic renal damage
• C. Postrenal obstruction
• D. Liver failure

Answer: (A)

When dehydration drops the circulating volume, renal perfusion falls. The kidneys respond by conserving water and nitrogen waste, so urea (BUN) is reabsorbed more than creatinine. This makes the BUN rise disproportionately compared with creatinine, producing a high BUN/creatinine ratio (often >20:1) with a relatively normal creatinine level. This pattern signals prerenal azotemia from decreased renal perfusion, rather than intrinsic kidney damage or an obstruction. Intrinsic renal damage would lift creatinine more and not show as high a BUN relative to creatinine, while obstruction and liver failure present with different patterns of lab changes.