Case History: Case H

A 73-year-old man presented with fever and lethargy. He was taking steroids for an autoimmune disease, and previous history included heart valve lesions. After treatment for Aspergillosis pneumonia, he gradually improved, but about a week later, he complained of weakness in his left hand and numbness in his right leg. A CT scan showed ring-enhancing lesions in the cerebral hemispheres. Despite treatment, he died 3 weeks later. What is the most likely diagnosis and what microscopic characteristics would be expected?