**Core Concept:** Nephrotoxicity is the harmful effect of a substance on the kidneys, leading to potential acute or chronic kidney injury. It can be due to drugs, chemicals, or other agents. Understanding nephrotoxicity helps in choosing the least harmful options for patients.

**Why the Correct Answer is Right:** Option C, "Acyclovir," is the least nephrotoxic among the given choices. Acyclovir is an antiviral drug primarily used for treating herpes simplex virus infections. It is selectively taken up by infected cells via the viral glycoprotein gE-gI complex, which reduces its concentration in the blood, thereby reducing the risk of nephrotoxicity.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Nitroglycerin):** Nitroglycerin is a vasodilator used in the management of angina pectoris and myocardial infarction. It can cause hypotension, which, in turn, reduces renal blood flow and potentially leads to nephrotoxicity.

B. **Option B (Amiodarone):** Amiodarone is an antiarrhythmic drug used in the treatment of life-threatening arrhythmias. It can cause nephrotoxicity due to its nephrotoxic effects, including tubular damage, interstitial nephritis, and acute kidney injury.

C. **Option C (Acyclovir):** As explained above, acyclovir is less nephrotoxic as it selectively concentrates in infected cells, reducing the systemic concentration and thus the risk of nephrotoxicity.

D. **Option D (Methotrexate):** Methotrexate is an immunosuppressive drug used in the treatment of various autoimmune disorders and cancer. It can cause nephrotoxicity due to its inhibitory effects on renal function and tubular secretion.

**Clinical Pearl:** When choosing medications for patients, consider nephrotoxicity as a potential adverse effect and choose the least nephrotoxic options. In this case, Acyclovir is the least nephrotoxic choice among the given options.