**Question:** An of the following can induce methemoglobinemia EXCEPT:

A. Nitroprusside
B. Nicotinamide adenine dinucleotide (NADH)
C. Methylene blue
D. Aspirin

**Core Concept:**
Methemoglobinemia is a condition characterized by an abnormal increase in methemoglobin levels in the blood, leading to impaired oxygen transport to tissues. Methemoglobin is a form of hemoglobin where the iron in the heme group is oxidized to methemoglobin. Normal levels of methemoglobin are typically around 1-3% and can be increased by various agents.

**Why the Correct Answer is Right:**
D) Aspirin does not directly induce methemoglobinemia. Although aspirin inhibits platelet aggregation and cyclooxygenase enzymes, it does not directly affect the heme group or cause oxidative stress, leading to methemoglobinemia.

**Why Each Wrong Option is Incorrect:**

A) Nitroprusside: Nitroprusside is a vasodilator medication that reduces blood pressure. It works by releasing nitric oxide (NO) to relax blood vessels and decrease peripheral resistance. One of its side effects is methemoglobinemia due to its direct interaction with the heme group and oxidizing iron to methemoglobin.

B) Nicotinamide adenine dinucleotide (NADH): NADH is an electron carrier in cellular respiration, playing a crucial role in energy production within the mitochondria. While it is involved in various cellular processes, NADH itself does not directly induce methemoglobinemia.

C) Methylene blue: Methylene blue is primarily used for its vasodilatory effects and as a selective serotonin-norepinephrine reuptake inhibitor. However, it is also a methemoglobin-inducing agent due to its direct interaction with the heme group and oxidizing iron to methemoglobin. This is the correct answer.

**Clinical Pearl:**

Methemoglobinemia can be caused by various factors such as drugs, chemicals, and environmental factors. Clinical presentation includes cyanosis, fatigue, and headache. Treatment involves reducing methemoglobin levels with methemoglobin reductase (like methylene blue) or using oxidoreductase (like الأزرق الكادوفيلي). Prevention involves understanding the risk factors and avoiding exposure to methemoglobin-inducing agents.