Methemoglobinemia causes hemoglobin unable to release oxygen.

• A. Methemoglobinemia
• B. Cyanide Poisoning
• C. Carbon Monoxide Poisoning
• D. Opioid Overdose

Answer: (A)

Oxygen delivery hinges on hemoglobin’s ability to pick up oxygen in the lungs and release it to tissues where it’s needed. In methemoglobinemia, iron in hemoglobin is oxidized to the ferric state (Fe3+), forming methemoglobin that cannot bind oxygen. This reduces the amount of functional hemoglobin available to carry O2. The remaining normal hemoglobin molecules tend to hold onto oxygen more tightly, shifting the oxygen-hemoglobin dissociation curve to the left, which makes it harder to unload oxygen to tissues. The result is tissue hypoxia even though arterial oxygen levels may be normal. Clinically, you may see cyanosis and symptoms of hypoxia with a normal PaO2. Other options don’t fit this pattern: cyanide prevents cells from using oxygen, carbon monoxide reduces oxygen delivery by binding hemoglobin, and opioid overdose mainly causes hypoventilation rather than a problem with oxygen release from hemoglobin.