The oxygen-hemoglobin dissociation curve shifts rightward (decreased O₂ affinity) in all of the following conditions EXCEPT:

• A Fever
• B Increased 2,3-DPG levels
• C Hypercarbia
• D Methemoglobinemia ✓

Explanation

Methemoglobinemia involves oxidation of Fe²⁺ to Fe³⁺, which cannot bind O₂ and also causes a left shift (increased O₂ affinity) of the remaining functional heme groups — the Haldane effect. This paradoxically reduces O₂ delivery despite tissue hypoxia. Fever, increased 2,3-DPG, and hypercarbia (Bohr effect — decreased pH) all cause rightward shifts, reducing Hb-O₂ affinity and facilitating O₂ unloading to tissues. Carbon monoxide poisoning also causes a left shift.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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