**Core Concept**
The 'A' pattern squint, also known as the 'A' pattern deviation, is a type of strabismus characterized by an inward and upward deviation of the eye. This condition is often associated with a specific muscle imbalance.

**Why the Correct Answer is Right**
The superior oblique muscle is primarily responsible for the 'A' pattern squint. This muscle is involved in the rotation of the eye, particularly in the intorsion (inward rotation) and depression (downward movement) of the globe. In individuals with an 'A' pattern squint, the superior oblique muscle is overactive, leading to an excessive intorsion and depression of the affected eye. This muscle imbalance causes the eye to deviate inward and upward, resulting in the characteristic 'A' pattern.

**Why Each Wrong Option is Incorrect**
**Option A:** The superior rectus muscle primarily controls the elevation (upward movement) of the eye and is not directly involved in the 'A' pattern squint.

**Option B:** The inferior rectus muscle is responsible for the depression (downward movement) of the eye, but it does not contribute to the 'A' pattern squint.

**Option D:** The inferior oblique muscle primarily controls the extorsion (outward rotation) of the eye and is not directly involved in the 'A' pattern squint.

**Clinical Pearl / High-Yield Fact**
In patients with an 'A' pattern squint, the cover-uncover test can be used to determine the direction of the deviation. When the cover-uncover test is performed, the eye with the 'A' pattern squint will typically move inward and upward.

**✓ Correct Answer: C. Superior oblique**