Multiple sclerosis (most common cause in young patients)
Stroke
Malignancy located in brainstem and 4th ventricle
CN III palsy would show additional deficit including weakness of elevation, ptosis, pupil dilation
Internuclear ophthalmoplegia
Multiple sclerosis 
Nystagmus
Stroke
Ocular palsy
Malignancy located in brainstem and 4th ventricle
Facial paralysis
CN III palsy would show additional deficit including weakness of elevation, ptosis, pupil dilation