MPO and Anxiety: RCS analysis confirmed a nonlinear association (p for nonlinearity < 0.01), with an inflection point at 30 ng/mL: below this threshold, each 1 ng/mL MPO increase reduced anxiety risk (OR = 0.982, CI: 0.970–0.994), while levels above it heightened risk (OR = 1.004, CI: 1.001–1.008).