In ureotelic animals, urea is formed by

Options

A. Ornithine cycle
B. Cori cycle
C. Krebs' cycle
D. EMP pathway

Explanation

### Step-by-Step Solution:

1. **Understanding Ureotelic Animals**:
- Ureotelic animals are those that excrete urea as their primary nitrogenous waste. Examples include mammals, marine fishes, and most terrestrial amphibians.

2. **Formation of Urea**:
- Urea is formed in the liver through a process known as the urea cycle or the ornithine cycle. This cycle is crucial for converting toxic ammonia, which is produced from the deamination of amino acids, into urea.

3. **Deamination of Amino Acids**:
- The first step in the urea formation involves the deamination of amino acids. During deamination, the amino group (-NH2) is removed from the amino acids, producing ammonia (NH3) and a corresponding keto acid.

4. **Entry into the Urea Cycle**:
- The ammonia produced from deamination is then converted into urea through a series of reactions in the urea cycle. The ammonia combines with carbon dioxide to form carbamoyl phosphate.

5. **Steps of the Urea Cycle**:
- Carbamoyl phosphate enters the urea cycle, combining with ornithine to form citrulline.
- Citrulline then combines with aspartate to form argininosuccinic acid, which is subsequently converted into arginine and fumarate.
- Arginine is then hydrolyzed to produce urea and ornithine, completing the cycle.

6. **Conclusion**:
- Thus, in ureotelic animals, urea is formed primarily from deaminated amino acids through the ornithine cycle, which is a series of biochemical reactions that convert ammonia into urea.

### Final Answer:
In ureotelic animals, urea is formed by the deamination of amino acids through the ornithine cycle.

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