Which condition results in hemoglobin that cannot release oxygen readily to tissues?

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

Answer: (C)

When iron in hemoglobin is oxidized to the ferric (Fe3+) state, hemoglobin becomes methemoglobin. This form cannot reversibly bind oxygen, so it directly lowers the blood’s oxygen-carrying capacity. With more methemoglobin present, less functional hemoglobin is available to transport O2. The remaining normal hemoglobin also tends to hold onto oxygen more tightly, shifting the oxygen dissociation curve to the left, which makes it harder for tissues to release the oxygen they need. The net result is tissue hypoxia despite adequate arterial oxygen levels. Clinically this presents with cyanosis and sometimes chocolate-brown blood that doesn’t improve with supplemental oxygen. Other listed conditions affect oxygen delivery through different routes: carbon monoxide also reduces oxygen unloading but by occupying hemoglobin with CO and shifting the curve; cyanide blocks cellular respiration; opioid overdose depresses respiration.