Inability to abduct left eye with LMN fascial palsy on same side. The lesion is in
**Core Concept**
The question assesses the student's understanding of the innervation of the extraocular muscles and the clinical implications of a lower motor neuron (LMN) fascial palsy. The abducens nerve (cranial nerve VI) is responsible for the abduction of the eye, and its dysfunction results in inability to abduct the eye on the same side.
**Why the Correct Answer is Right**
The abducens nerve arises from the pons and courses laterally to innervate the lateral rectus muscle, which is responsible for the abduction of the eye. In the presence of a LMN fascial palsy, the lesion is likely to be located in the peripheral nerve or its branches, leading to weakness or paralysis of the muscles innervated by the affected nerve. In this case, the inability to abduct the left eye suggests a lesion affecting the left abducens nerve.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the nucleus of the abducens nerve is located in the pons, and a lesion in this area would result in an internuclear ophthalmoplegia (INO), characterized by impaired adduction of the eye on the same side.
**Option B:** This option is incorrect because the oculomotor nerve (cranial nerve III) is responsible for the majority of the extraocular muscles, including the medial rectus, which is involved in adduction of the eye, not abduction.
**Option C:** This option is incorrect because the trochlear nerve (cranial nerve IV) is responsible for innervating the superior oblique muscle, which is involved in intorsion and depression of the eye, not abduction.
**Clinical Pearl / High-Yield Fact**
A LMN fascial palsy of the abducens nerve can be caused by a variety of conditions, including stroke, trauma, and tumors. The clinical hallmark of this condition is the inability to abduct the eye on the same side, which can be assessed by the "abduction test" or "duane test".
**Correct Answer: C. The lesion is in the right pons or the left abducens nerve.**