Professional and Linguistic Assessments Board (PLAB) Practice Exam

What causes diplopia in a patient with oculomotor nerve palsy?

1. Weakness in facial muscles

2. Irregularities in the retina

3. Misalignment of the eyes

4. Increased intracranial pressure

The correct answer is: Misalignment of the eyes

In the case of oculomotor nerve palsy, diplopia, or double vision, occurs primarily due to misalignment of the eyes. The oculomotor nerve is responsible for innervating several muscles that control eye movement, including those that adjust the position of the eye in various directions. When there is a lesion or dysfunction of this nerve, the muscles it innervates become weak or paralyzed. This weakness leads to an inability of the affected eye to cooperate with the other eye during gaze shifts, resulting in a situation where the two eyes do not point in the same direction. As a result, when the patient looks at an object, the brain receives two different images, leading to diplopia. Understanding this mechanism highlights the importance of the oculomotor nerve in maintaining proper eye alignment and coordination. In this specific case, the other options do not directly cause diplopia related to oculomotor nerve palsy; for example, weakness in facial muscles is not primarily responsible for eye misalignment, irregularities in the retina pertain to visual disturbances rather than eye positioning, and increased intracranial pressure typically leads to various symptoms but does not directly cause muscle weakness or misalignment related to oculomotor nerve function.