A 40 -year-old woman has relapsing/remitting multiple sclerosis. She has experienced two attacks of disease previously that have left her with residual lower extremity weakness. She was initially treated with glucocoicoids with some improvement in her symptoms. However, she is currently only able to walk with a rolling walker about 100 m. A prior antibody test shows that she is negative for reactivity to the JC virus. You are planning to sta a disease-modifying therapy. Which of the following drug that causes a maximum reduction in appearance of new lesions and change in disease severity ?