Which scenario best represents pre-renal acute kidney injury?

• A. Acute tubular necrosis from nephrotoxin exposure
• B. Severe dehydration causing decreased renal perfusion
• C. Urinary tract obstruction from a stone
• D. Chronic kidney disease from hypertension

Answer: (B)

Pre-renal acute kidney injury happens when there isn’t enough blood flow reaching the kidneys, so filtration drops even though the kidney tissue itself isn’t damaged. Severe dehydration lowers the circulating volume, which reduces renal perfusion and the glomerular filtration rate. The kidney responds by trying to conserve sodium and water to preserve blood volume, so urine is typically concentrated and the kidneys reabsorb most electrolytes, which is reflected in a low fractional excretion of sodium and an elevated BUN relative to creatinine.

This scenario fits best because the root problem is decreased perfusion from volume depletion, not direct tubule injury, an obstruction, or a chronic kidney condition. Acute tubular necrosis from a nephrotoxin involves intrinsic kidney damage to the tubules; urinary tract obstruction from a stone causes post-renal failure due to outflow blockage; chronic kidney disease from hypertension is a gradual, long-standing loss of nephrons rather than an acute drop in perfusion.