1. Figure 15a. PMLprogressive multifocal leukoencephalopathy in a 65-year-old woman with multiple medical problems, no known immunosuppressive disease, and rapid neurologic decline over 2 months. (a) Axial T2-weighted MR image shows confluent asymmetric T2 hyperintensity in the cerebral white matter, without substantial mass effect, involving the subcortical U-fibers (arrows). (b) Axial contrast-enhanced T1-weighted MR image shows a small focal enhancement (arrow) corresponding to a biopsy site but otherwise no enhancement of the bilateral white matter areas. (c) Gross autopsy specimen shows confluent right frontal demyelination (arrows). (d) Photomicrograph shows that SV40 antibody stain is positive in glial nuclei infected with JC virus (arrow), the causative agent of PMLprogressive multifocal leukoencephalopathy. (SV40 antibody stain; original magnification, x200.)