E-cigarette use avoids CO, lowering carboxyhaemoglobin levels

Steps:

• Cigarette smoke contains carbon monoxide (CO), which binds to haemoglobin forming carboxyhaemoglobin.
• E-cigarette vapour lacks CO, so no such binding occurs.
• After a month of e-cigarette use, CO exposure is eliminated.
• Carboxyhaemoglobin concentration decreases due to absence of CO.

Why C is correct:

• Carboxyhaemoglobin is the complex of haemoglobin and CO (HbCO), directly formed by CO inhalation, absent in e-cigarettes.

Why the others are wrong:

• A. Carbaminohaemoglobin forms from CO2 binding to haemoglobin, unaffected by CO absence.
• B. Carbonic anhydrase is an enzyme catalyzing CO2 hydration, not impacted by CO levels.
• D. Hydrogencarbonate ions result from CO2 transport in blood, unrelated to CO exposure.

Final answer: C