**Core Concept:** The occulomotor nerve (CN III) is one of the 12 cranial nerves in the human body. It plays a crucial role in controlling eye movements, pupillary dilation, and some extraocular muscles. The rectus muscles are responsible for the six extraocular muscles that move the eye in different directions.

**Why the Correct Answer is Right:** The rectus muscles are supplied by the corresponding cranial nerves. The rectus muscles involved in horizontal eye movements (right and left) are supplied by the oculomotor nerve (CN III). The rectus muscles involved in vertical eye movements (up and down) are supplied by the trochlear nerve (CN IV). Therefore, the rectus muscle that is not supplied by the occulomotor nerve is the one involved in vertical eye movements.

**Why Each Wrong Option is Incorrect:**

A. Occulomotor nerve (CN III) supplies the rectus inferior muscle, which is involved in downward eye movement (infraduction). This option is incorrect because it is the rectus superior muscle (CN IV) that is supplied by the trochlear nerve (CN IV), responsible for upward eye movement (supination).

B. The rectus inferior muscle is also involved in the downward eye movement (infraduction). This option is incorrect as mentioned above regarding the involvement of CN IV (trochlear nerve) in the rectus inferior muscle.

C. This option is incorrect because the rectus inferior muscle is still supplied by the occulomotor nerve (CN III) and is involved in downward eye movement (infraduction).

D. The rectus medialis muscle is involved in adduction (drawing the eyeball towards the midline) and is supplied by the oculomotor nerve (CN III). This option is incorrect because it is the rectus medialis muscle that is supplied by the oculomotor nerve (CN III) and is involved in adduction.

**Clinical Pearl:** Understanding the anatomy and innervation of extraocular muscles is crucial for diagnosing and treating disorders affecting eye movements, such as ophthalmoplegia or ptosis. Knowledge of these nerves allows for proper clinical examination and differential diagnosis of various neurological disorders affecting eye movements.