What is the nephron's primary role in urine formation?

• A. To filter blood and form urine.
• B. To regulate blood glucose.
• C. To package proteins.
• D. To produce hormones.

Answer: (A)

The key idea is that urine formation starts with filtering blood and then selectively processing that filtrate along the nephron to produce urine. Blood plasma is pushed through the glomerular filtration barrier, creating a filtrate that contains water and small solutes but excludes large proteins and cells. As this filtrate flows through the tubules, essential substances such as glucose, amino acids, and most ions and water are reabsorbed back into the bloodstream, while wastes and any excess ions are secreted into the tubular fluid. This coordinated filtration, reabsorption, and secretion transforms the filtrate into urine and helps maintain fluid and electrolyte balance. Other choices describe functions that aren’t the nephron’s main job in urine production—glucose regulation, protein packaging, or hormone production—so they don’t capture the primary role of forming urine.