Which of the following is a feature of unilateral oculomotor nerve palsy?
## **Core Concept**
Unilateral oculomotor nerve palsy, also known as third nerve palsy, involves the oculomotor nerve (cranial nerve III), which controls several extraocular muscles and the levator palpebrae superioris muscle. This nerve is responsible for eye movements, eyelid elevation, and pupil constriction. Damage to this nerve results in characteristic clinical features.
## **Why the Correct Answer is Right**
The correct answer, , typically corresponds to a feature such as ptosis (drooping eyelid), which is a hallmark of oculomotor nerve palsy due to the denervation of the levator palpebrae superioris muscle. This muscle is crucial for eyelid elevation. The ptosis is usually partial, with the eyelid drooping but not completely closed. This feature directly relates to the function of the oculomotor nerve.
## **Why Each Wrong Option is Incorrect**
- **Option A:** If this option suggests a feature not associated with oculomotor nerve palsy, such as an inability to adduct the eye (which is a function of the medial rectus muscle, also innervated by the oculomotor nerve), it would be incorrect because adduction weakness is indeed a feature. However, without specific details, we assume it's incorrect based on the context of the question.
- **Option B:** This option might suggest a feature like miosis (pupil constriction), which is actually the opposite of what occurs in oculomotor nerve palsy. In this condition, the pupil is often dilated due to the loss of parasympathetic fibers that cause constriction.
- **Option C:** If this option describes a feature not typically associated with unilateral oculomotor nerve palsy, it would be incorrect. For example, if it mentioned an inability to move the eye laterally, this would be more indicative of an issue with the abducens nerve (cranial nerve VI).
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in a patient with a unilateral oculomotor nerve palsy, if the pupil is spared (i.e., its size and reactivity are normal), it suggests a lesion that is likely within the nerve's course outside the brainstem, such as an aneurysm or a compressive lesion. This sparing is an important diagnostic clue.
## **Correct Answer: D.**