This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuro Ophthalmology, Subject name: Ophthalmology
Which of the following investigation is not necessary for investigating optic neuritis? 
Select and explain. 
A. MRI head and orbit, B. ESR, C. USG B scan, D. Visual Fields
C: USG B scan
Answer: C: USG B scan 
Explanation: 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.