Cranial Nerves One to Six Case 3

A 28-year-old woman comes into the ED complaining of blurred vision in her left eye, which has developed over the last hour or so. She does not usually have any eye problems and there is no history of trauma.
Her eye movements are painful and she is complaining of a generalised headache. She has been unwell with a viral illness for a couple of days and has a low grade pyrexia.
Which nerve is involved and what is the likely diagnosis?

Can you identify the possible causes of this presentation?

Select one or more options.

Which investigation is most likely to give you a diagnosis?