Final answer:
The nucleus is the structure that is not present in the axon of a neuron.
Step-by-step explanation:
The structure not present in the axon of a neuron is the Nucleus. While other cellular components such as the Golgi apparatus, endoplasmic reticulum, and mitochondria are present in the cell body or soma, the nucleus is found only within the soma and does not extend into the axon. The axon is responsible for carrying electrical signals away from the cell body to other neurons or effector cells.