The correct answer is option 2, the liver.

When there is an excess of amino acids in the body, they undergo a process called deamination in the liver. Deamination is the removal of the amino group from the amino acids, resulting in the formation of ammonia. Ammonia is a toxic substance that needs to be converted into a less toxic form, which is urea.

The liver plays a vital role in this conversion process. It converts the ammonia into urea through a series of reactions called the urea cycle. The urea is then transported to the kidneys through the bloodstream and excreted from the body in the urine.

The kidneys, which are mentioned in option 1, do play a role in the excretion of urea, but they are not responsible for the breakdown of excess amino acids into urea. The spleen, mentioned in option 3, and the rectum, mentioned in option 4, do not have any significant involvement in this process.

In summary, the liver is responsible for breaking down excess amino acids into urea through the urea cycle.