Which extraocular rectus muscle has its insertion site CLOSEST to the limbus?

The medial rectus muscle is the correct answer because it has its insertion point closest to the limbus, which is the border between the cornea and the sclera of the eye. The medial rectus is positioned on the nasal side of the eye and serves to adduct the eyeball, moving it toward the nose. Its anatomical location allows it to insert very close to this limbal area, which is crucial for various eye movements.

In contrast, the lateral rectus muscle inserts on the temporal side of the eye, while the inferior and superior rectus muscles insert vertically on the inferior and superior portions, respectively. These muscles' insertions are further from the limbus comparatively. Therefore, the unique positioning of the medial rectus muscle plays a significant role in its function and proximity to the limbus.