The correct answer is:

trochlear nerve (CN IV)

Educational Objective:

List the signs and symptoms of a trochlear nerve lesion.

Key Point:

A trochlear nerve lesion results in the inability to move the eye down and medially.

Explanation:

The trochlear nerve (CN IV) innervates the superior oblique muscle, which rotates and moves the eye medially and down (toward 5 o’clock if the eye were a clock face). The Latin word trochlea translates as "pulley." One can think of dysfunction of the trochlear nerve (CN IV) as tipping the head laterally to the opposite (contralateral) side of the affected eye in an attempt to see their inferior and medial environment with an eye that cannot move straight down or down and medially (BOTH of these movements require the superior oblique, with the straight down movement being a combination of the superior oblique and inferior rectus working together; whereas the down and medial movement is accomplished by the superior oblique alone – see the second figure below). 

Essentially, the head rotates the malfunctioning eye (and by default the other eye as well) to a 5 o’clock position. 

Pearl:

The inferior rectus and medial rectus cannot combine to compensate for a paralyzed superior oblique muscle. Notice that the inferior rectus by itself moves the eye down and laterally, not just straight down.

Follow this link to a detailed discussion of an isolated trochlear nerve palsy (updated in 2021): Cranial Nerve 4 Palsy

Review the following figures: