<prompt> Which of the following investigation is not necessary for investigating optic neuritis?MRI head and orbitESRUSG B scanVisual Fields  <response>: USG B scan is not useful for evaluating optic neuritis. MRI head and orbit is needed for evaluation of optic neuritis to rule out MS. ESR- sed in infectious causes of optic neuritis. Visual fields are used for follow up and extent of vision loss. OCT Optical coherence tomography- used to see amount of optic nerve edema. 