<prompt> If tyrosine level in blood is normal without external supplementation, deficiency of which of the following is ruled out ?TryptophanPhenylalanineHistidineIsoleucine  <response>: Ans. is b i.e., PhenylalanineTyrosine is synthesized from phenylalanine.In phenylalanine deficiency or in disorders in which phenylalanine cannot be conveed into tyrosine phenylketonuria, tyrosine becomes an essential amino acid and should be supplemented from outside. 