A 66 year-old male with facial nerve palsy has weakness in both upper and lower portions of the right side of his face. What best describes the motor neuron involved?

The correct choice indicates that the motor neuron involved is a right lower motor neuron. In facial nerve palsy, also known as Bell's palsy when idiopathic, the facial nerve (cranial nerve VII) is compromised. Because this patient demonstrates weakness in both the upper and lower portions of the face on the right side, it suggests that there is a direct lesion affecting the facial nerve on that side.

Lower motor neuron lesions are characterized by weakness or paralysis on the same side of the body as the lesion occurs since they directly innervate the muscles. The involvement of both the upper and lower parts of the face indicates that the entire facial nerve is affected, which is consistent with a lower motor neuron lesion. In contrast, upper motor neuron lesions typically spare the upper facial muscles due to bilateral cortical innervation, leading to contralateral weakness affecting only the lower face.

This understanding allows us to determine that the location of the motor neuron injury must be on the same side as the paralysis, which aligns with the designation of the right lower motor neuron as the affected structure in this specific case.