(2R,3S)-2-Amino-3-(3,4-dihydroxyphenyl)-3-hydroxypropanoic acid; Droxidopa Enantiomer; D-Threo-Dihydroxyphenylserine; (+)-threo-3,4-Dihydroxyphenylserine; D-Tyrosine, β,3-dihydroxy-, threo-; (βS)-β,3-Dihydroxy-D-tyrosine; D-Tyrosine, β,3-dihydroxy-, (βS); D-threo-3,4-Dihydroxyphenylserine; Droxidopa Impurity-C