The correct answer to this question is C. Cranial nerve IV is used to look down and are in the medial position. The cranial nerve IV innervates the superior oblique muscle. If this cranial nerve is damaged on one side, double vision will occur, resulting in the inability to walk down the stairs.
Although a lesion in the cervical spinal cord ventral horn cells could make it hard to walk down stairs it would not make the patient have double vision. A lesion in the cranial nerve VII may affect the face, but it would not cause the patient to have double vision or not allow them to walk down stairs.
A lesion in the cranial nerve VI would affect the eyes, but they would not cause the patient to have double vision or make it difficult to walk down stairs. A lesion in the cranial nerve III would only cause short-term problems for the patient and would not cause them to see double or would not allow them to go down stairs.
Cranial nerve iv -to walk down stairs, one has to have the ability to move the eyes down when they are in the medial position. this involves the use of cranial nerve iv (trochlear nerve), which innervates the superior oblique muscle (whose action is to pull the eye downward when in the medial position). if there is damage to this nerve on one side, the eyes will not be able to focus on the same visual field, thus producing double vision. cranial nerve iv is classified as a general somatic efferent fiber because it innervates skeletal muscle and it is derived from somites.