Final answer:
The etiologic agent of progressive multifocal leukoencephalopathy (PML) is a virus called the JC virus.
Step-by-step explanation:
The etiologic agent of progressive multifocal leukoencephalopathy (PML) is a virus. PML is caused by the reactivation of the JC virus, which is a type of human polyomavirus. The JC virus targets and infects oligodendrocytes, a type of brain cell responsible for the production and maintenance of myelin.