The reabsorption of which of the following substances primarily occurs in the proximal tubule?

The proximal tubule of the nephron plays a crucial role in the reabsorption process, where a significant amount of vital substances is reclaimed from the filtrate back into the bloodstream. This section of the nephron is responsible for roughly 65-70% of the sodium ions filtered through the glomerulus. As sodium ions are reabsorbed, water follows passively due to osmosis, resulting in substantial water reabsorption as well.

Additionally, urea, a waste product, is also reabsorbed in the proximal tubule, though to a lesser extent compared to sodium and water. The uptake of urea contributes to the kidney's ability to concentrate urine and maintain the body's nitrogen balance.

Given this information, it is clear that not only sodium but also water and urea undergo significant reabsorption in the proximal tubule. Therefore, the correct understanding is that all these substances—sodium ions, urea, and water—are primarily reabsorbed in this part of the nephron, making a comprehensive understanding of renal physiology essential for interpreting the roles of various components in kidney function.