A patient presents to the doctor with diplopia when looking towards the right. On examination, he is unable to move his right eye laterally past the midline. Damage to which of these nerves would produce this clinical presentation?
**Core Concept**
The abducent nerve (cranial nerve VI) is responsible for controlling the lateral rectus muscle, which abducts the eye. Damage to this nerve results in inability to move the eye laterally past the midline, leading to diplopia (double vision) when looking in the direction of the affected muscle.
**Why the Correct Answer is Right**
The abducent nerve contains preganglionic parasympathetic fibers that synapse in the ciliary ganglion and postganglionic fibers that innervate the sphincter pupillae and ciliary muscles. However, its primary function is to innervate the lateral rectus muscle, which is responsible for outward eye movement. When the abducent nerve is damaged, the patient is unable to move the affected eye laterally past the midline, resulting in diplopia. This is a classic presentation of abducens nerve palsy.
**Why Each Wrong Option is Incorrect**
**Option B:** The trochlear nerve (cranial nerve IV) is responsible for innervating the superior oblique muscle, which is involved in downward and inward eye movement. Damage to the trochlear nerve would result in difficulty moving the eye downward and inward, not laterally.
**Option C:** The optic nerve (cranial nerve II) is responsible for transmitting visual information from the eye to the brain. Damage to the optic nerve would result in loss of vision, not diplopia due to lateral eye movement.
**Option D:** The oculomotor nerve (cranial nerve III) is responsible for innervating several extraocular muscles, including the medial rectus, superior rectus, inferior rectus, and inferior oblique. Damage to the oculomotor nerve would result in difficulty moving the eye in multiple directions, not just laterally.
**Clinical Pearl / High-Yield Fact**
A classic presentation of abducens nerve palsy is the "abducens palsy sign," where the patient is unable to move the affected eye laterally past the midline, resulting in diplopia. This is a critical sign to recognize in clinical practice.
**✓ Correct Answer: A. Abducent nerve**