Final answer:
The breakdown of complex waste takes place in the kidney because the kidneys are responsible for excreting waste products produced by metabolism. These waste products are filtered out of the blood by nephrons inside the kidneys.
Step-by-step explanation:
The breakdown of complex waste takes place in the kidney because the kidneys are responsible for excreting waste products produced by metabolism, such as urea and uric acid. These waste products are filtered out of the blood by tiny units called nephrons inside the kidneys. If the kidneys did not remove these waste products, they would build up in the blood and damage the body.