**Core Concept**
Paralytic squint, also known as paralytic strabismus, occurs when a muscle controlling eye movement is weakened or paralyzed, resulting in impaired binocular vision and diplopia (double vision). This condition is often caused by cranial nerve palsies, particularly those affecting the oculomotor, trochlear, or abducens nerves.

**Why the Correct Answer is Right**
In paralytic squint, the affected extraocular muscle becomes unable to move the eye in its normal direction, leading to misalignment of the eyes and diplopia. This is because the brain cannot integrate the images from both eyes correctly, resulting in double vision. The severity of diplopia can vary depending on the extent of muscle weakness or paralysis. For instance, if the lateral rectus muscle is paralyzed, the affected eye will be unable to abduct (move outward), causing double vision when looking laterally.

**Why Each Wrong Option is Incorrect**
**Option B:** Non-paralytic squint, also known as non-paralytic strabismus, occurs when the eyes are misaligned due to other factors such as refractive errors, congenital conditions, or accommodative problems. While non-paralytic squint can also cause diplopia, it is not the primary cause of diplopia as seen in paralytic squint.

**Option C:** Both paralytic and non-paralytic squint can cause diplopia, but the question asks for the usual cause, which is paralytic squint.

**Option D:** None of the above is incorrect because paralytic squint is indeed a common cause of diplopia.

**Clinical Pearl / High-Yield Fact**
A classic way to remember the muscles affected by cranial nerve palsies is to associate them with their corresponding cranial nerves: oculomotor nerve (III) - superior rectus, inferior rectus, medial rectus, and inferior oblique; trochlear nerve (IV) - superior oblique; and abducens nerve (VI) - lateral rectus.

**✓ Correct Answer: A. Paralytic squint**