A 43-year-old woman has had a headache and fever for the past 2 weeks following a severe respiratory tract infection accompanying bronchiectasis. On physical examination, her temperature is 38.3degC. There is no papilledema. She has no loss of sensation or motor function, but there is decreased vision in the left half of her visual fields. CT scan of the head shows a sharply demarcated, 3-cm, a ring-enhancing lesion in the right occipital region. A lumbar puncture is done, and laboratory analysis of the CSF shows numerous leukocytes, increased protein, and normal glucose levels. What is the most likely diagnosis?