IDUA and Parkinson disease: Observational studies have consistently shown that serum levels of vitamin D and urate are inversely associated with the risk of PD.30 However, MR analyses have not supported a causal effect of either of these biomarkers on PD risk, suggesting that these observational associations may be confounded or attributable to reverse causation.30,31 MR analyses have also been used to identify novel risk factors of PD, such as plasma levels of alpha-l-iduronidase (IDUA).