Where in the kidney does mannitol primarily exert its action?

Mannitol primarily exerts its action in the proximal convoluted tubule and the descending loop of Henle. As an osmotic diuretic, mannitol increases osmotic pressure within the renal tubules, which leads to the retention of water in the tubular fluid. In the proximal convoluted tubule, where a significant amount of sodium and water reabsorption occurs, mannitol prevents this reabsorption due to its osmotic properties, thus promoting diuresis.

Similarly, in the descending loop of Henle, mannitol can also inhibit water reabsorption by increasing osmolarity, preventing the reabsorption of water that normally occurs in this segment. This mechanism highlights the way mannitol enhances the excretion of water, leading to increased urine production.

The locations mentioned in the other options do not align with the primary site of mannitol’s action. For instance, while the distal convoluted tubule and collecting duct are involved in later stages of urine formation, mannitol's efficacy is mainly observed earlier in the nephron structure. Additionally, it does not act directly at the afferent arterioles, which are responsible for regulating blood flow into the glomerulus rather than affecting osmotic