A client is prescribed vitamin B12 for pernicious anemia. Which finding indicates the medication is having a therapeutic effect?

A. Paresthesia only in the hands and around the mouth: Paresthesia is a symptom of vitamin B12 deficiency and usually improves with effective treatment. Continued or new-onset paresthesia suggests the medication is not yet effective or that neurological damage may persist. B. Increase in the mean corpuscular volume (MCV): An elevated MCV is a hallmark of untreated macrocytic anemia, such as in B12 deficiency. A decrease, not increase, in MCV is expected as therapy becomes effective and red blood cell production normalizes. C. Decrease in the serum vitamin B12 level: Therapeutic vitamin B12 supplementation should increase serum B12 levels. A decrease would suggest poor absorption or noncompliance with therapy and would not indicate treatment success. D. Increase in the client's activity tolerance: Improved energy levels and reduced fatigue are clinical signs that vitamin B12 therapy is effective. As anemia resolves, oxygen delivery improves, allowing greater physical activity and stamina.