Which condition primarily affects the brain or spinal cord parenchyma?

Encephalitis is the correct choice as it primarily affects the brain parenchyma, which consists of the functional tissue of the brain involved in various neuronal processes. This condition is characterized by inflammation of the brain itself, which can be caused by viral infections, autoimmune responses, or other factors. Symptoms of encephalitis can vary but often include fever, confusion, seizures, and neurological deficits due to the inflammation directly impacting brain tissue.

In contrast, meningitis primarily involves the membranes (meninges) surrounding the brain and spinal cord rather than the parenchyma itself. Neurosyphilis, a manifestation of syphilis, affects the nervous system but can impact various areas, often involving more than just the parenchyma. Multiple sclerosis is an autoimmune disease that affects the central nervous system, leading to demyelination and damage but does not specifically refer to immediate inflammation of the brain tissue as seen in encephalitis. Thus, encephalitis is distinguished by its direct effect on brain parenchyma, making it the most appropriate answer to the question.