Whether it is allowed or not is up to the exam's administrator. The relevant law in the US, the Americans with Disabilities Act, does not require the admin to

Whether it is allowed or not is up to the exam's administrator. The relevant law in the US, the Americans with Disabilities Act, does not require the admin to allow it unless the exam taker provides a letter from a health-care professional asserting a disability and reimburses the admin for any extra costs in accommodating the disability.