Q. A 37-year-old woman came to your clinic with gradually progressive insomnia and forgetfulness for the past three months. She also complains of occasional numbness and tingling in her fingers. She says climbing stairs is frustrating for her. She has been on a vegan diet since adolescence and does not remember the last time she received a vitamin supplement. She has no significant medical history. She does not use tobacco, alcohol, or illicit drugs. She is alert and cooperative. Her body mass index is 18.5 kg/m2. The blood pressure is 113/67 mmHg, the pulse rate is 88/min, and the temperature is 37.6 degrees Celsius (99.68 degrees Fahrenheit). She is pale. The sensory and motor exams of the fingers are normal. The rest of the examination is not remarkable. The lab tests show macrocytic normochromic anemia (Hb=9.2 g/dL, MCV=112 fL). Other parameters in the CBC test are normal. The iron panel is normal. Serum vitamin B12 level is 280 pg/mL (normal level >300 pg/mL), and serum folate level is 3.8 (normal level >4 ng/mL). The serum levels of methylmalonic acid and homocysteine are both increased. What is the necessary treatment for her?