A 35-year-old female has progressive numbness of the right arm and difficulty seeing objects in the left visual field. She is known to be HIV positive,but has not consistently taken medications in the past. On examination,she appears healthy,but has a right homonymous hemianopsia and decreased sensory perception in her left upper extremity and face. Her CD4 count is 75 cells per fA,and her MRI is consistent with a demyelinating lesion of the left parietooccipital area. CSF PCR for JC virus is positive. Which of the following is the most appropriate treatment in this case