Final answer:
Ornithine is not produced directly from arginine.
Step-by-step explanation:
The compound that is not produced directly from arginine is D) Ornithine.
Arginine is cleaved into ornithine and urea in the urea cycle through the action of the enzyme arginase. Ornithine is then regenerated in the cycle, while urea is formed and released into the blood.
However, creatine, nitric oxide, and urea are all compounds that are produced directly from arginine.