A leg!
You should know that the human meridians are spread throughout the body, connecting all parts!
If the body is incomplete, or there is some abnormality, then the meridians within will also exhibit missing sections, displacements, and other very serious issues!
And with incomplete or abnormal meridians, mana cannot circulate properly, so how can one absorb spiritual energy, increase mana, and enhance cultivation? All these factors are closely related!
After all, most cultivation techniques involve the transport of spiritual energy and the circulation of the meridian cycle, all of which pass through the eight extraordinary meridians and the twelve regular meridians. It's just that the methods differ and the minor meridians they traverse are also distinct.
Of course, such situations are not without a solution.
One just needs to find special cultivation techniques to practice, or consume corresponding rare and precious treasures.