In which of the following conditions would patients have an increased concentration of methemoglobin?

a. Patient receiving drug containing sulfonamide
b. Patient exposed to reducing agents
c. Patient with hemoglobin S (Hb S)
d. Patient with methemoglobin reductase deficiency

ANS: D
A patient with methemoglobin reductase deficiency will have an elevated concentration of methemoglobin. It is normal for some small amount of hemoglobin to be oxidized on a daily basis. However, this does not build up in the RBCs because the iron is constantly being reduced to its functional ferrous state. When this enzyme is deficient, methemoglobin increases in the RBCs. Rare patients taking sulfonamide-containing drugs may develop sulfhemoglobin. Oxidizing agents can increase hemoglobin M (Hb M), but not reducing agents. Patients with Hb S have normal levels of methemoglobin.