Professional and Linguistic Assessments Board (PLAB) Practice Exam

Which cranial nerve is responsible for the movement causing diplopia when going down stairs?

1. CN V - Trigeminal

2. CN III - Oculomotor

3. CN IV - Trochlear

4. CN VI - Abducens

The correct answer is: CN IV - Trochlear

The correct answer pertains to the role of the trochlear nerve (CN IV) in ocular movement. The trochlear nerve is specifically responsible for innervating the superior oblique muscle of the eye, which is crucial for downward and lateral eye movements. When an individual is descending stairs, the ability to properly coordinate eye movements is vital for maintaining clear vision and depth perception. Diplopia, or double vision, can occur if there is a dysfunction in the trochlear nerve, as this may result in impaired function of the superior oblique muscle. This leads to difficulties in adduction and downward gaze, hence the double vision when looking down stairs. The other cranial nerves listed also play important roles in eye movement. The oculomotor nerve (CN III) innervates several extraocular muscles, which contributes to movements like elevation, adduction, and depression of the eye, but it is primarily the trochlear nerve that is most directly linked to the symptom of diplopia when specifically moving downwards. The abducens nerve (CN VI) controls lateral eye movement, impacting peripheral vision but not directly involved in the downward motion associated with stair descent. The trigeminal nerve (CN V), while involved in sensation, does not directly control