**Core Concept**
Exotropia is a form of strabismus characterized by outward deviation of one or both eyes due to a misalignment of the extraocular muscles. The etiology of exotropia can be congenital, acquired, or idiopathic. Understanding the underlying pathophysiology is crucial for diagnosing and managing this condition.

**Why the Correct Answer is Right**
Third nerve palsy, specifically oculomotor nerve (cranial nerve III) dysfunction, is a common cause of exotropia. The oculomotor nerve innervates the medial rectus, inferior rectus, and superior rectus muscles, which are responsible for eye adduction, depression, and elevation, respectively. When the oculomotor nerve is damaged, the affected eye may exhibit outward deviation due to unopposed action of the lateral rectus muscle. This results in exotropia, particularly if the third nerve palsy affects the medial rectus muscle.

**Why Each Wrong Option is Incorrect**
**Option B:** Optic neuritis typically presents with visual loss, pain on eye movement, and a relative afferent pupillary defect, but it is not directly associated with exotropia. Optic neuritis affects the optic nerve, which transmits visual information from the retina to the brain, whereas exotropia is a result of extraocular muscle misalignment.

**Option C:** Abducens injury affects the lateral rectus muscle, leading to esotropia (inward deviation) rather than exotropia. The abducens nerve (cranial nerve VI) innervates the lateral rectus muscle, which is responsible for eye abduction.

**Option D:** Papilloedema is an increase in intracranial pressure that causes swelling of the optic disc. While papilloedema can lead to visual disturbances, it is not a direct cause of exotropia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with exotropia, it is essential to assess the extraocular muscles and their innervation. A thorough neurological examination can help identify underlying causes, such as third nerve palsy, which may require prompt treatment to prevent amblyopia and other complications.

**✓ Correct Answer: A. Third nerve palsy**