**Core Concept**
Amphotericin B is a polyene antifungal medication that has a narrow therapeutic index. Its toxicity is primarily related to its ability to bind to cell membranes and cause potassium efflux, leading to hypokalemia and other electrolyte imbalances.

**Why the Correct Answer is Right**
The correct answer is Foscarnet. Foscarnet is a potassium-sparing diuretic that can be used to counteract the hypokalemia caused by Amphotericin B. It works by inhibiting the sodium-potassium ATPase pump in the kidneys, thus reducing potassium excretion. This can help mitigate the nephrotoxic effects of Amphotericin B.

**Why Each Wrong Option is Incorrect**
**Option A:** Mannitol is an osmotic diuretic that can worsen hypokalemia and is not effective in reducing Amphotericin B toxicity.
**Option B:** Furosemide is a loop diuretic that can cause further potassium loss and is not a suitable choice for reducing Amphotericin B toxicity.
**Option C:** Sodium bicarbonate may be used to manage acidosis associated with Amphotericin B, but it does not directly reduce toxicity.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor potassium levels closely when administering Amphotericin B and consider using potassium-sparing diuretics like Foscarnet to mitigate hypokalemia.

**Correct Answer:** C. Foscarnet.