**Core Concept**
Mannitol is an osmotic diuretic used to reduce intraocular pressure (IOP) by creating an osmotic gradient. This **osmotic effect** helps in decreasing the fluid volume in the eye. The underlying principle involves the movement of water out of the eye due to the osmotic gradient created by mannitol.

**Why the Correct Answer is Right**
Mannitol works by increasing the osmotic pressure of the blood, which in turn draws water out of the eye, thereby reducing the IOP. This is achieved through the **osmotic diuresis** mechanism, where mannitol is not reabsorbed by the kidneys, creating an osmotic effect that helps in reducing the fluid volume.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it doesn't relate to the mechanism by which mannitol decreases IOP.
**Option B:** Incorrect as it is unrelated to the osmotic effect of mannitol.
**Option C:** Incorrect as it doesn't describe the correct mechanism of action of mannitol in reducing IOP.
**Option D:** Incorrect because, although not specified, typically mannitol's effect is not primarily through this mechanism.

**Clinical Pearl / High-Yield Fact**
Mannitol is often used in acute settings to reduce IOP, such as in acute glaucoma. It's crucial to remember that mannitol acts quickly but has a short duration of action.

**Correct Answer:** D. Osmotic gradient.