Koro syndrome appears most frequently in Southeast Asia and produces a devastating physical certainty: those who suffer from it feel, day after day, that their genitals are shrinking and being drawn into the body. The fear is not symbolic. It is bodily, progressive, and in documented cases it is accompanied by extreme panic at the idea of dying from that retraction.
It is not the only case in which the mind rewrites the reality of the body. In alien hand syndrome, the person sees their own hand move, pick up objects, or act without any conscious command. They recognize it as their own, but swear they do not control it. And in Cotard syndrome, the subject looks in the mirror, sees their reflection, and even so maintains with total conviction that they are dead or decomposing.
These are real neurological and psychiatric disorders, documented in clinical literature, although they are rare. Science is still investigating why the brain can construct such extreme certainties about the body itself.