- In human metabolism, the primary form through which excess nitrogen is eliminated is urea.
- This process takes place via the urea cycle, also known as the ornithine cycle, which occurs mainly in the liver.

- When proteins are broken down, amino groups are released as free ammonia, which is toxic to the body.
- To safely eliminate this excess nitrogen, the body converts ammonia into urea, a less toxic, water-soluble compound.
- Urea is then transported in the blood to the kidneys, where it is excreted in urine.

Let's clarify the options:
- Ammonia is the initial toxic nitrogenous waste but is not the form eliminated because of its toxicity.
- Uric acid is the final product of purine metabolism and is excreted mainly by birds and reptiles; in humans, excessive uric acid can cause gout.
- Urea is the main nitrogenous waste product in mammals, including humans, and is the correct answer.
- Creatinine is a breakdown product of creatine phosphate in muscle but is not involved in nitrogen excretion through the urea cycle.

Thus, the body's primary mechanism for nitrogen removal is by converting ammonia to urea in the liver, which is then safely excreted by the kidneys.

Reference: Lehninger Principles of Biochemistry, 7th Edition, Chapter 27: Nitrogen Metabolism and the Urea Cycle