Which type of acute kidney injury is caused by decreased renal perfusion?

• A. Intrinsic AKI
• B. Prerenal AKI
• C. Postrenal AKI
• D. Chronic kidney disease

Answer: (B)

Reduced blood flow to the kidneys is the key idea here. When the kidneys aren’t getting enough perfusion, the glomerular filtration rate drops even though the kidney tissue itself isn’t damaged yet. This is prerenal acute kidney injury, a problem of supply rather than injury to the kidney itself. The body’s response to low perfusion is to conserve water and sodium to try to maintain circulating volume, so the urine becomes concentrated and the kidneys reabsorb much of the filtered fluid.

In practical terms, prerenal AKI often shows a high BUN relative to creatinine (BUN/Cr ratio typically >20), low urine sodium (<20 mEq/L), a fractional excretion of sodium under 1%, and high urine osmolality (>500 mOsm/kg). These patterns reflect intact tubular function with preserved reabsorptive capacity, because the issue is the reduced delivery of blood to the kidneys, not damage inside the kidneys.

This contrasts with intrinsic AKI, where the kidney tissue itself is damaged (for example, tubular injury), and postrenal AKI, where obstruction to urine flow causes backpressure. Chronic kidney disease is a long-standing decline in kidney function, not an acute result of perfusion loss. If perfusion is restored, prerenal AKI often improves quickly, which highlights why the decreased renal perfusion scenario is categorized as prerenal.