Acute renal failure is characterized by which feature?

• A. Gradual onset with gradual improvement
• B. Sudden onset with decreased urine output
• C. Chronic, slowly progressive course
• D. No urinary changes

Answer: (B)

Acute renal failure, now called acute kidney injury, involves a rapid decline in kidney function that develops over hours to days, often with a marked decrease in urine production. This sudden drop in urine output, or oliguria, is the hallmark feature because it shows the kidneys are abruptly failing to filter waste and maintain fluid balance. This contrasts with a gradual, long-term decline seen in chronic kidney disease, which evolves slowly over months to years. So the option describing a sudden onset with decreased urine output best captures the typical presentation of acute renal failure. (Note: some cases can be non-oliguric early on, but the abrupt change with reduced urine output is the classic concept tested.)