Testing this patient's anti–N-methyl-D-aspartate receptor (NMDAR) antibodies will confirm her diagnosis. This young woman presents with a subacute alteration in mental status over several weeks, characterized by psychiatric symptoms, seizure activity, and involuntary movements. These features, although individually nonspecific, suggest NMDAR encephalitis when combined. A recently described entity, it is one of the most common autoimmune encephalitides, especially in children and young adults, although it is infrequently diagnosed in older adults. Patients with anti-NMDAR encephalitis classically present with psychiatric symptoms, which can vary from agitation to frank psychosis. Other common signs include autonomic instability, choreoathetoid movements, and seizures or status epilepticus. This syndrome was initially recognized in young women with teratomas but has subsequently been diagnosed in patients without an obvious antigenic trigger, such as tumor or infection. Laboratory confirmation requires detection of anti-NMDAR antibodies in serum or cerebrospinal fluid (CSF).

Borrelia burgdorferi is the bacterium that causes Lyme disease. The most common neurologic presentations include cranial neuropathy, meningitis, radiculopathy, and encephalomyelitis, which are inconsistent with this patient's presentation.

Nuchal skin biopsy for immunohistochemistry or polymerase chain reaction may be used to diagnose rabies. Rabies often presents with localized paresthesias at the site of inoculation, with inexorable progression to coma and death over days to weeks. Autonomic instability, muscle spasm, and hydrophobia are commonly present.

VDRL testing of CSF is an indirect test for neurosyphilis. This spirochetal illness causes neurologic symptoms similar to Lyme disease. Early in the disease course, meningitis and cranial neuropathies predominate; however, in late neurosyphilis, dementia or tabes dorsalis may be seen. These symptoms are inconsistent with those distinguishing anti-NMDAR encephalitis.