A 58-year-old woman comes to the physician for evaluation of worsening fatigue for 1 week. She also has a 1-year history of hand pain and stiffness. Four months ago, she started a new medication for these symptoms. Medications used prior to that included ibuprofen, prednisone, and hydroxychloroquine. Examination shows a subcutaneous nodule on her left elbow and old joint destruction with boutonniere deformity. Her hemoglobin concentration is 10.1 g/dL, leukocyte count is 3400/mm³, and platelet count is 101,000/mm³. Methylmalonic acid levels are normal. Which of the following could have prevented this patient's laboratory abnormalities?

1. A)Vitamin B6
2. B)Vitamin B12
3. C)Amifostine
4. D)2-Mercaptoethanesulfonate
5. E)LeucovorinGABARITO