The relationship that exists when an organism lives in or on another organism and feeds off that organism is called

It is called Parasitism.

Explanation:

Parasitism is symbiotic relationship where an organisms lives in or on it's host thereby feeding and causing harm on it host. Examples of such relationship is parasites like roundworm, ticks, fleas that feed on humans and cause harm.

thrombocytopenia, which is a deficiency in blood platelets, can cause bleeding and bruising in the dermis similar to senile purpura. other possible causes include: exposure to ultraviolet rays and aging skin are common causes of senile purpura. senile purpura may also be a sign of collagen loss in the skin and bones.