## Core Concept
Mannitol is an osmotic diuretic that works by increasing the osmotic pressure of the blood, thereby reducing fluid movement from tissues into the bloodstream and promoting urine production. It is primarily used to reduce intracranial pressure and to treat cerebral edema.

## Why the Correct Answer is Right
The correct answer involves the use of mannitol in conditions where reducing intracranial pressure or managing cerebral edema is crucial. Mannitol acts by creating an osmotic gradient that draws water out of brain cells, thereby reducing brain swelling. This mechanism makes it particularly useful in acute settings such as head trauma, stroke, or brain surgery.

## Why Each Wrong Option is Incorrect
- **Option A:** While mannitol can be used in certain cases of poisoning to help remove toxins, its primary and most well-known use is not for poisoning treatment.
- **Option B:** Mannitol is not typically used for treating hypertension; its use is more specific to conditions involving increased intracranial pressure.
- **Option C:** Although mannitol can increase urine output, it is not primarily used for treating urinary retention.

## Clinical Pearl / High-Yield Fact
A key point to remember is that mannitol's effect can be monitored by measuring urine output and serum osmolality. A critical clinical correlation is that mannitol can cause a rebound increase in intracranial pressure if not used carefully, especially if it leads to a rapid shift of water back into brain tissues.

## Correct Answer Line
**Correct Answer: D.**