Uric Acid
Serum urate, the end product of purine metabolism, and an unusually clean example of a test that is definitive for one condition and speculative for everything else. It directly causes gout through crystal deposition in joints. Its observational associations with hypertension, metabolic syndrome and cardiovascular disease are a different proposition entirely.
Mendelian randomisation studies have largely failed to confirm a causal role for uric acid in cardiovascular outcomes, suggesting the observational association reflects confounding by obesity, alcohol, kidney function and diet rather than a direct causal pathway. Clearly useful for diagnosing and monitoring gout; not validated as a standalone cardiovascular or longevity risk marker, and no guideline recommends treating asymptomatic hyperuricaemia to reduce cardiovascular risk.