**Core Concept**
Amphotericin B is a polyene antifungal medication that binds to ergosterol in fungal cell membranes, leading to cell lysis and death. However, its use is limited by several unwanted effects.

**Why the Correct Answer is Right**
One of the primary unwanted effects of Amphotericin B is nephrotoxicity, which can lead to acute kidney injury or worsening renal function in susceptible individuals. This is due to the drug's ability to cause vasoconstriction and increase vascular resistance in the kidneys, ultimately leading to reduced glomerular filtration rate. The drug also causes oxidative stress and inflammation in the renal tissue, exacerbating the nephrotoxic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** While Amphotericin B can cause hypokalemia, it is not the most significant or characteristic unwanted effect of the drug.
**Option B:** Amphotericin B is not primarily associated with QT prolongation or cardiac arrhythmias; other antifungal agents like fluconazole are more commonly linked to these effects.
**Option C:** Although Amphotericin B can cause fever and chills, these are more common during the initial infusion and are not the most significant unwanted effects of the drug.

**Clinical Pearl / High-Yield Fact**
To minimize nephrotoxicity, Amphotericin B should be infused slowly (over 2-3 hours), and patients should be well-hydrated before and during treatment. Monitoring of renal function and electrolytes is essential during therapy.

**Correct Answer: D. Nephrotoxicity**