**Core Concept**
Methemoglobinemia is a condition characterized by an abnormal amount of methemoglobin in the blood, which is a form of hemoglobin that cannot bind oxygen. This can lead to tissue hypoxia and is often caused by drugs that interfere with the normal function of hemoglobin or reduce the amount of functional hemoglobin.

**Why the Correct Answer is Right**
Methemoglobinemia can be caused by various drugs, including local anesthetics, such as benzocaine and lidocaine. These drugs can oxidize hemoglobin, converting it to methemoglobin. This is due to the presence of a nitrobenzene or nitroso group in these compounds, which can accept electrons from the iron in hemoglobin, leading to its oxidation. Other drugs that can cause methemoglobinemia include nitroglycerin, nitric oxide donors, and certain antibiotics, such as sulfonamides and dapsone.

**Why Each Wrong Option is Incorrect**
**Option A:** Nitrous oxide does not typically cause methemoglobinemia, although it can displace oxygen from hemoglobin.
**Option B:** Furosemide, a loop diuretic, does not cause methemoglobinemia. Its mechanism of action involves inhibiting the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle.
**Option C:** Metformin, an oral antidiabetic agent, is not typically associated with methemoglobinemia. Its mechanism of action involves decreasing hepatic glucose production and increasing insulin sensitivity.

**Clinical Pearl / High-Yield Fact**
Methemoglobinemia can be treated with methylene blue, which acts as a reducing agent to convert methemoglobin back to normal hemoglobin. However, methylene blue should be used with caution in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, as it can cause hemolysis in these patients.

**Correct Answer: C. Metformin is not typically associated with methemoglobinemia.