Which of the following is an unmodifiable risk factor for Parkinson's disease?

• A. Well water
• B. Industrial metals
• C. Low estrogen
• D. Age over 40

Answer: (D)

Age is an unmodifiable risk factor because you can’t change how old you are, and the likelihood of Parkinson’s disease increases as people get older. As time passes, cumulative neural stress, mitochondrial changes, and protein misfolding contribute to greater vulnerability of dopaminergic neurons, so incidence rises with age—often becoming more apparent after 60, though the risk begins to rise earlier.

In contrast, the other options reflect exposures or conditions that can be reduced or managed. Well water can carry environmental toxins that people might limit by using different water sources or filtration. Industrial metals exposure can be mitigated with safety measures and protective practices. Hormonal factors like estrogen could be modified in some contexts with medical management. Thus, the only inherently unmodifiable factor listed here is age over 40, making it the unmodifiable risk factor.