A patient is unable to move eye outward beyond midline . The lesion is in
**Core Concept**
The patient's inability to move the eye outward beyond the midline is indicative of a lateral rectus muscle palsy, which is caused by a lesion affecting the sixth cranial nerve (abducens nerve) or its innervation.
**Why the Correct Answer is Right**
The abducens nerve is responsible for innervating the lateral rectus muscle, which is the primary muscle responsible for outward gaze. A lesion affecting this nerve or its innervation will result in an inability to move the eye outward beyond the midline. This is due to the loss of motor function to the lateral rectus muscle, which is essential for abducting the eye.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because a lesion affecting the superior rectus muscle, which is innervated by the third cranial nerve (oculomotor nerve), would result in difficulty elevating the eye, not moving it outward.
* **Option B:** This is incorrect because a lesion affecting the medial rectus muscle, which is innervated by the third cranial nerve, would result in difficulty moving the eye inward, not outward.
* **Option C:** This is incorrect because a lesion affecting the trochlear nerve (fourth cranial nerve), which innervates the superior oblique muscle, would result in difficulty moving the eye downward and inward, not outward.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that lateral rectus muscle palsy is often caused by a lesion affecting the abducens nerve, which can be due to a variety of factors including trauma, tumors, vascular events, or intracranial pressure.
**Correct Answer:** C.