A 40-year-old woman presented to the emergency department with a two-day history of pain in her right eye, worsening with eye movements, and blurred vision in the right eye. She had no floaters or eye redness and did not have recent trauma. Her ocular and medical history were unremarkable, and she was not taking medications.

On physical examination, the patient appeared well and had normal vital signs. Her best corrected visual acuity was 20/50 in the right eye and 20/20 in the left eye. The intraocular pressures were normal at 13 mm Hg. The patient had a relative afferent pupillary defect in her right eye, and her extraocular movements were full. There was no proptosis or periorbital swelling. Findings on slit-lamp examination were normal, and funduscopic examination with pupil dilation showed a normal macula, peripheral retina, and optic nerve. Results of a full neurologic examination were otherwise normal.

What is the most appropriate next step?

A. Order complete blood count, antinuclear antibody test, and venereal disease research laboratory test.

B. Diagnose optic neuritis and arrange for nonurgent magnetic resonance imaging (MRI) of the brain with ophthalmology follow-up.

C. Obtain computed tomography of the orbits to rule out compressive lesion of the optic nerve.

D. Perform lumbar puncture to rule out infectious causes and for oligoclonal band screening of the cerebrospinal fluid (CSF).