What specific phenomenon causes a slow rotation of Mercury's elliptical path that cannot be fully explained by Newtonian gravity alone?

Answer

Precession

Mercury's proximity to the Sun results in deviations explained by general relativistic effects, causing a slow rotation of the elliptical path itself, a phenomenon termed precession.

What specific phenomenon causes a slow rotation of Mercury's elliptical path that cannot be fully explained by Newtonian gravity alone?
Spaceastronomyorbitplanetpath