USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What term describes the reabsorption of fluid along the proximal tubule in which there is no change in tubular osmolality?

1. Balance among GFR, peritubular oncotic pressure, and proximal tubular reabsorption

2. Renal artery stenosis

3. Isosmotic reabsorption

4. Metanephric mesenchyme interaction

The correct answer is: Isosmotic reabsorption

The term that describes the reabsorption of fluid along the proximal tubule without any change in tubular osmolality is isosmotic reabsorption. In the proximal tubule, approximately two-thirds of the filtered sodium and water are reabsorbed. This process occurs in a manner that maintains the osmolarity of the tubular fluid, meaning that as water is reabsorbed, an equivalent amount of solute (such as sodium) is reabsorbed simultaneously. This isosmotic reabsorption is a crucial function of the kidneys, as it helps preserve the osmotic balance necessary for proper fluid homeostasis in the body. The reabsorption process is primarily driven by sodium transport mechanisms and osmotic gradients, ensuring that the osmolarity remains stable despite the loss of fluid volume in the tubular system. In contrast, the other terms presented do not relate directly to this specific process. For example, balance among GFR, peritubular oncotic pressure, and renal artery stenosis pertain to different aspects of renal hemodynamics and do not specifically describe the phenomenon of isosmotic reabsorption. Metanephric mesenchyme interaction refers to developmental processes in kidney formation and is not relevant in this context.