This may depend on the person entirely. Some people might have autism, HPPD, visual eye syndrome and musical ear syndrome all at once, while mental retardation is based on many obscure factors.
Mental retardation is a developmental disability that first appears in children under the age of 18. It is defined as an intellectual functioning level (as measured by standard tests for intelligence...
Single gene defects such as phenylketonuria (PKU) and other inborn errors of metabolism may also cause mental retardation if they are not found and treated early.