Final answer:
Decreased tone is not a symptom of upper motor neuron syndrome; it is associated with lower motor neuron lesions, making 'C. Decreased tone' the correct answer to the question.
Step-by-step explanation:
The question of which of the following is NOT a symptom of upper motor neuron (UMN) syndrome can be addressed by understanding the typical symptoms of UMN lesions. Symptoms include clonus, hyperactive deep reflexes, weakness, and increased muscle tone, often termed spasticity. However, decreased tone is not a symptom associated with UMN lesions; it is more commonly seen in lower motor neuron (LMN) syndrome, where there may be a lack of muscle tone known as hypotonicity or flaccidity. Accordingly, the correct response to the question is 'C. Decreased tone', as all other options listed are symptoms of UMN syndrome.