Final answer:
A person with a buildup of waste is likely having issues with the excretory system, with the kidneys being a major organ responsible for waste removal.
Step-by-step explanation:
A person whose body fails to effectively remove waste products, leading to a buildup of harmful substances, is most likely experiencing impaired function of the excretory system. The kidneys are a major organ of this body system. The excretory system plays a critical role in maintaining homeostasis by removing waste products and excess water from the body through the urinary system, skin (sweat), lungs (carbon dioxide), and large intestine.