**Core Concept**
Osmotic diuresis is a mechanism of increased urine production caused by the presence of osmotically active substances in the renal tubules. These substances, such as mannitol, glucose, or urea, increase the osmotic pressure within the tubules, preventing water reabsorption and leading to increased urine output. This process is a key component of the body's response to certain pathologies, such as diabetes mellitus.

**Why the Correct Answer is Right**
During osmotic diuresis, the presence of osmotically active substances in the renal tubules prevents the reabsorption of water, leading to an increase in urine production. This is because the osmotically active substances increase the osmotic pressure within the tubules, effectively "locking in" water and preventing it from being reabsorbed. As a result, the concentration of the urine is decreased, and the volume of urine produced is increased.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the mechanism of osmotic diuresis. While the presence of osmotically active substances in the renal tubules does lead to increased urine production, it is not due to decreased blood volume.
* **Option B:** This option is incorrect because it is a description of the mechanism of isotonic diuresis, not osmotic diuresis. Isotonic diuresis is caused by an increase in blood volume, not the presence of osmotically active substances in the renal tubules.
* **Option C:** This option is incorrect because it is a description of the mechanism of pressure diuresis, not osmotic diuresis. Pressure diuresis is caused by an increase in blood pressure, not the presence of osmotically active substances in the renal tubules.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that osmotic diuresis is often seen in patients with diabetes mellitus, where the presence of glucose in the renal tubules leads to increased urine production. This can result in a significant loss of water and electrolytes, leading to dehydration and other complications.

**Correct Answer:** C.