**Core Concept**
The superior oblique muscle is one of the six extraocular muscles responsible for controlling eye movements. It plays a crucial role in the rotation of the eyeball, particularly in the inferior and abduction movements. The superior oblique muscle is innervated by the trochlear nerve (CN IV), which is the thinnest cranial nerve.

**Why the Correct Answer is Right**
The primary action of the superior oblique muscle is to rotate the eyeball inferiorly, which is achieved through its insertion on the sclera of the eyeball. When the superior oblique muscle contracts, it pulls the eyeball in a downward direction, thereby facilitating the movement of the eyeball from the superior to the inferior position. This action is essential for the proper functioning of the extraocular muscles, particularly during movements such as reading or looking down.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the superior rectus muscle is responsible for elevating the eyeball, not rotating it inferiorly.
**Option B:** This option is incorrect because the medial rectus muscle is responsible for adducting the eyeball, not rotating it inferiorly.
**Option C:** This option is incorrect because the inferior oblique muscle is responsible for rotating the eyeball superiorly, not inferiorly.

**Clinical Pearl / High-Yield Fact**
The superior oblique muscle is often affected in cases of trochlear nerve palsy, which can lead to difficulties in downward gaze. This can be a significant clinical finding in patients with neuro-ophthalmic disorders.

**Correct Answer: D. Inferior rotation of the eyeball.**