You are seeing a 60-year-old female in a primary care setting who has presented with hearing loss. On questioning this patient informs you that the hearing loss is in her right ear only and came on suddenly a week ago. The patient is not complaining of pain or tinnitus and is systemically well. On examination you note that the tympanic membrane is intact and there is no redness in the ear drum. This patient also has difficulty balancing. What is the most important diagnosis to rule out?