Urea is formed from ammonia and carbon dioxide in .......

Explanation

**Step-by-Step Solution:**

1. **Understanding the Reaction**: Urea is formed from the reaction of ammonia (NH₃) and carbon dioxide (CO₂). This reaction can be represented as:
\[ \text{NH}_3 + \text{CO}_2 \rightarrow \text{Urea} \]

2. **Identifying the Cycle**: The process of urea formation from ammonia and carbon dioxide is part of a biochemical pathway known as the urea cycle. This cycle is also referred to as the ornithine cycle.
...