Which of the following is a function of the kidney?

• A. Excretion of metabolic wastes
• B. Regulation of red blood cell production
• C. Activation of vitamin D in liver
• D. Production of insulin

Answer: (A)

The kidneys’ defining function is to remove waste products from the blood and maintain the body's internal balance by forming urine. This excretion process, which handles metabolic wastes like urea, creatinine, and uric acid, is what keeps chemical levels, fluid, and acid-base status in check. While the kidneys do influence red blood cell production by releasing erythropoietin, the direct, primary job described in this option is the elimination of wastes from the body. The idea that the liver activates vitamin D is inaccurate because the kidney—but not the liver—converts vitamin D to its active form. Insulin is produced by the pancreas, not the kidney. Therefore, excretion of metabolic wastes best captures the kidney’s function.