A. Incorrect response: See explanation C.
B. Incorrect response: See explanation C.
C. Correct response: Methemoglobin (metHb) results when the iron atom in heme is oxidized from Fe2+ (ferrous ion) to Fe3+ (ferric ion). In the ferric state (oxidized form), iron is incapable of binding with O2, and the affinity of other heme groups for O2 increases (i.e., shifts the oxyhemoglobin curve to the left). The normal methemoglobin blood level (<2%) may be due in part to metabolism of endogenous NO. Methemoglobin reductase within erythrocytes converts endogenously produced methemoglobin to normal hemoglobin. Methemoglobinemia is uncommon at the NO doses used for therapeutic (20 ppm) inhalation. A few cases of methemoglobinemia have been reported in association with iNO therapy, generally with high doses of iNO (e.g., 80 or more ppm). In patients with decreased methemoglobin reductase, methemoglobinemia will be more likely.
D. Incorrect response: See explanation C.