DL-4-Hydroxymandelic acid monohydrate is an organic compound used primarily as a chiral building block and intermediate in the synthesis of pharmaceuticals, agrochemicals, and other fine chemicals. It serves as a substrate in biochemical research for studying enzyme activities, such as mandelate racemase, involved in microbial metabolism of aromatic compounds. In analytical applications, it functions as a reference standard for high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and other methods to detect and quantify hydroxyphenylacetic acid derivatives in biological fluids. While not a primary biomarker, its levels can be assessed in urine or plasma for investigating disorders of tyrosine and phenylalanine catabolism, such as tyrosinemia or hepatic dysfunction.