Which finding is most characteristic of a lower motor neuron lesion?

• A. Hyperreflexia
• B. Babinski sign
• C. Fasciculations
• D. Spasticity

Answer: (C)

Lower motor neuron lesions affect the final common pathway to muscle, so the usual result is flaccid weakness with decreased tone and diminished or absent reflexes, often accompanied by muscle atrophy. A classic, visible sign here is fasciculations—twitches caused by spontaneous firing of motor units after denervation. This contrasts with upper motor neuron signs, which come from disruption of cortical or pyramidal tract control and include hyperreflexia, spasticity, and the Babinski sign. Fasciculations reflect peripheral nerve or motor neuron instability, whereas the other signs listed reflect central (upper motor neuron) control loss.