Final answer:
The nurse should monitor the hemoglobin level to determine the therapeutic effectiveness of folic acid in treating megaloblastic anemia because it reflects the oxygen-carrying capacity of the blood, which is a key factor affected by this condition.
Step-by-step explanation:
Megaloblastic anemia is characterized by the presence of large, abnormal, immature red blood cells (megaloblasts) due to a deficiency of vitamin B12 and/or folate. The most appropriate laboratory value to monitor in this case would be the hemoglobin level, as an increase in hemoglobin levels would indicate an improvement in the condition and therefore the therapeutic effectiveness of the folic acid. Although a complete blood count (CBC) would include other values such as the white blood cell count, platelet count, and red blood cell count, the hemoglobin level is directly related to the oxygen-carrying capacity of the blood, which is often compromised in anemia.