All of the following ocular muscles are supplied by CN III EXCEPT:
## Core Concept
The question tests knowledge of the cranial nerve (CN) innervation of the extraocular muscles. CN III, also known as the oculomotor nerve, is responsible for innervating several muscles that control eye movements. Understanding the specific muscles innervated by CN III and those innervated by other cranial nerves is crucial for diagnosing and managing ocular motility disorders.
## Why the Correct Answer is Right
The oculomotor nerve (CN III) supplies the following muscles:
- **Medial rectus**: involved in adduction of the eyeball
- **Superior rectus**: primarily responsible for elevation, with secondary actions of adduction and intorsion
- **Inferior rectus**: primarily responsible for depression, with secondary actions of adduction and extorsion
- **Inferior oblique**: involved in extorsion, elevation, and abduction of the eyeball
The **Lateral rectus muscle** is the one not supplied by CN III; instead, it is innervated by CN VI, the abducens nerve, which is responsible for abduction of the eyeball.
## Why Each Wrong Option is Incorrect
- **Option A:** The **Medial rectus** is supplied by CN III.
- **Option B:** The **Inferior rectus** is also supplied by CN III.
- **Option C:** The **Inferior oblique** muscle receives its innervation from CN III as well.
## Clinical Pearl / High-Yield Fact
A useful clinical correlation to remember is the "3, 4, 6" rule for cranial nerve innervation of the extraocular muscles: CN III (3) supplies the medial rectus, superior rectus, inferior rectus, and inferior oblique; CN IV (4) supplies the superior oblique; and CN VI (6) supplies the lateral rectus. This helps in quickly identifying which nerves are responsible for the actions of the extraocular muscles.
## Correct Answer: D. Lateral rectus.