**Core Concept**
Paralytic squint, also known as paralytic strabismus, is a type of strabismus caused by weakness or paralysis of one or more extraocular muscles. This leads to misalignment of the eyes and impaired binocular vision. The primary deviation in paralytic squint is the angle of deviation between the two eyes when the patient is fixating on a target.

**Why the Correct Answer is Right**
When a patient develops paralytic squint, the affected muscle is weakened, resulting in an inability to move the eye in the direction of action of that muscle. The primary deviation is the angle of deviation when the patient is fixating with the unaffected eye. This deviation is usually greatest when the patient is looking in the direction of action of the affected muscle, and it decreases when the patient looks in the opposite direction. The primary deviation is a result of the imbalance in the forces acting on the two eyes, leading to misalignment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the primary deviation is not the maximum angle of deviation, but rather the angle of deviation when the patient is fixating with the unaffected eye.
**Option B:** This option is incorrect because the secondary deviation is the angle of deviation when the patient is fixating with the affected eye, not the primary deviation.
**Option C:** This option is incorrect because the hypertrophic angle is the angle between the visual axis and the direction of action of the hypertrophic muscle, not the primary deviation in paralytic squint.

**Clinical Pearl / High-Yield Fact**
In paralytic squint, the primary deviation is greatest when the patient is looking in the direction of action of the affected muscle. This is because the weakened muscle is unable to generate sufficient force to move the eye in that direction, resulting in a greater deviation.

**Correct Answer:** D