**Core Concept**
The yoke muscle concept is used to describe the conjugate movement of extraocular muscles. The yoke muscle of an extraocular muscle is the muscle that moves in the same direction as the primary muscle, ensuring coordinated eye movement.

**Why the Correct Answer is Right**
The superior oblique muscle is responsible for intorsion, depression, and abduction of the eye. Its yoke muscle must therefore move in the same direction to maintain conjugate eye movement. The yoke muscle of the right superior oblique is the **left inferior oblique**. This is because the inferior oblique muscle also causes intorsion, depression, and abduction of the eye, and is therefore conjugate with the superior oblique muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** The left superior rectus is not the yoke muscle of the right superior oblique, as it causes elevation rather than depression.
**Option B:** The left medial rectus is not the yoke muscle of the right superior oblique, as it causes adduction rather than abduction.
**Option C:** The left inferior rectus is not the yoke muscle of the right superior oblique, as it causes extorsion rather than intorsion.

**Clinical Pearl / High-Yield Fact**
Remember that the yoke muscle concept is essential for understanding conjugate eye movement and the coordination of extraocular muscles.

**Correct Answer: D. Left inferior oblique**