Which description matches passive tubular reabsorption?

• A. Filtration at the glomerulus
• B. Osmosis causing water removal into blood
• C. Passive tubular reabsorption: water moves from tubules into blood, increasing the concentration of remaining solutes
• D. Active transport of solutes into blood

Answer: (C)

Passive tubular reabsorption involves water moving from the tubular lumen into the blood along osmotic gradients, without using energy. As water exits, the tubular fluid becomes more concentrated with the remaining solutes, reflecting that reabsorption of water leaves solutes behind and raises their concentration. This process is driven by osmosis through channels like aquaporins and occurs alongside reabsorption of solutes, but it does not require ATP. Filtration at the glomerulus is the movement of fluid into the nephron, not reabsorption, and active transport of solutes into blood is energy-dependent, not passive.