Typically, names ending in the letter a add an n to the end to make an adjective, while other vowels add an an.
... Arachnodactyly ataxia cataract aminoaciduria mental retardation[?] Arachnodactyly mental retardation dysmorphism[?] Arachnoid cysts[?] Arachnoiditis[?] Arakawa'sa ...