During which stage does water reabsorption initiate in the nephron?

Water reabsorption in the nephron initiates primarily in the proximal convoluted tubule. This segment is crucial for the reabsorption of approximately 65-70% of the filtrate's volume. During this process, water is reabsorbed through osmosis alongside solutes that are actively reabsorbed, such as sodium, glucose, and amino acids. This concurrent absorption of solutes creates an osmotic gradient that facilitates the passive reabsorption of water, making the proximal convoluted tubule a vital site for maintaining fluid balance in the body.

Other parts of the nephron, such as the distal convoluted tubule and the collecting duct, also participate in water reabsorption, but they are regulated to a greater extent by hormones like antidiuretic hormone (ADH) and aldosterone. The Loop of Henle contributes significantly to the concentration of urine but is not the primary site for the initial reabsorption of water. Therefore, it is in the proximal convoluted tubule where water reabsorption first begins, making it the correct answer.