A common disease like multiple sclerosis (MS) usually dominates the arena of demyelinating disorders; however, when a red flag such as a first relative family history of neurologic disease other than MS is present, the diagnostics may be more challenging. Recently, we have come across an intriguing clinical and diagnostic dilemma requiring extensive literature search and finally, decision making. Namely, initial presentation in our patient was that of a frequent condition such as optic neuritis and demyelinating CNS lesions, and then unexpected additional information during the usual diagnostic process, in the matter of first relative being diagnosed with a rare disease - Anderson-Fabry disease, oriented us in another direction. Almost paradoxically, the rare diagnosis being genetically confirmed, initiated treatment first. Furthermore, multitude of published data instigated clinical, radiological and laboratory uncertainty in our minds urging us to use a "wait and see" approach for the potential second diagnosis in order to prevent causing harm or labeling our patient wrongly by a potential misdiagnosis. However, further clinical course, a follow-up MRI, comparison with initial findings, and multi-disciplinary consultation safely directed us to a frequent diagnosis of multiple sclerosis. Our patient is currently treated for both conditions.