**Question:** All of the following drugs can induce Methemoglobinemia, except -

A. Nitroprusside
B. Nitroglycerin
C. Nitrofurantoin
D. Nitroxynil

**Correct Answer:** D. Nitroxynil

**Core Concept:**
Methemoglobinemia is a condition characterized by an increase in methemoglobin levels in the blood, leading to decreased oxygen carrying capacity. Methemoglobin is a form of hemoglobin in which the iron in the heme group is in its ferric (Fe3+) state, preventing it from binding to oxygen effectively.

**Why the Correct Answer is Right:**
Nitroxynil is a nitrite analogue that can convert the iron in the heme group of hemoglobin from ferrous (Fe2+) to ferric (Fe3+) state, leading to an increase in methemoglobin levels. This mechanism is similar to nitroglycerin and nitroprusside, which are also nitrite derivatives.

**Why Each Wrong Option is Incorrect:**

A. Nitroprusside: It is a nitric oxide donor, inducing a fall in blood pressure by dilating blood vessels. Although it can cause methemoglobinemia, this is a rare side effect.

B. Nitroglycerin: Similar to nitroxynil, nitroglycerin is a nitrite derivative that converts hemoglobin to methemoglobin. However, it is commonly associated with methemoglobinemia as a side effect.

C. Nitrofurantoin: Nitrofurantoin is a furoxone derivative used as an antibiotic. It is not a nitrite derivative and does not increase methemoglobin levels, making it safe from causing methemoglobinemia.

**Clinical Pearl:**
While methemoglobinemia can be a concerning side effect of certain medications, it is essential for medical students to understand the nuances among different drugs causing this condition. Recognizing these differences can help in appropriate prescribing and monitoring to prevent this potentially life-threatening complication.