**Question:** Methemoglobinemia may be caused by following, except:

A. Oxidation of iron in the heme group of hemoglobin to methemoglobin
B. Hemolysis of red blood cells
C. Hemolysis caused by oxidative stress
D. Decreased NADH and NADPH levels

**Core Concept:** Methemoglobinemia is a condition characterized by an increased proportion of methemoglobin (oxidized form of hemoglobin) in the blood, leading to reduced oxygen carrying capacity. This can be caused by various factors, including oxidation of iron in the heme group of hemoglobin, hemolysis, oxidative stress, and decreased availability of NADH and/or NADPH.

**Why the Correct Answer is Right:** The correct answer is **D.** Decreased NADH and NADPH levels. NADH and NADPH are essential coenzymes involved in the maintenance of the reduced state of the heme groups in hemoglobin and myoglobin, preventing the formation of methemoglobin. When NADH and NADPH levels are reduced, the heme groups may become oxidized, leading to the formation of methemoglobin.

**Why Each Wrong Option is Incorrect:**

A. **A.** Oxidation of iron in the heme group of hemoglobin to methemoglobin is the primary cause of methemoglobinemia. This option is incorrect because it describes the main mechanism behind the condition.

B. **B.** Hemolysis of red blood cells does not directly cause methemoglobinemia. Hemolysis leads to the release of heme and bilirubin, which can cause indirect methemoglobinemia (hemolysis-induced methemoglobinemia). This option is incorrect because it is an indirect cause rather than the primary cause.

C. **C.** Hemolysis caused by oxidative stress can lead to methemoglobinemia indirectly by releasing heme and bilirubin. This option is incorrect because it is an indirect cause, not the primary cause.

**Clinical Pearl:** Methemoglobinemia can be caused by various factors, including certain medications, chemicals, and genetic disorders. Understanding the primary cause (oxidation of iron in the heme group) helps differentiate between primary and secondary causes and guides appropriate treatment strategies.