DL-4-Hydroxy-3-methoxymandelic acid, also known as vanillylmandelic acid (VMA), is primarily used in clinical diagnostics as a biomarker for the detection and monitoring of neuroendocrine tumors, particularly pheochromocytoma and neuroblastoma. It is a metabolite of catecholamines, such as epinephrine and norepinephrine, and its elevated levels in urine can indicate excessive production of these hormones, often associated with such tumors. The compound is analyzed through laboratory tests, such as high-performance liquid chromatography (HPLC) or immunoassays, to assess its concentration. Additionally, it plays a role in research studies focused on understanding catecholamine metabolism and its implications in cardiovascular and neurological disorders. Its measurement is crucial for diagnosing and managing conditions linked to abnormal catecholamine levels.