**Core Concept**
Diplopia, also known as double vision, is a symptom characterized by the perception of two images of a single object. It is caused by the misalignment of the eyes, either due to problems with the muscles that control eye movement or the nerves that transmit signals between the eyes and the brain.

**Why the Correct Answer is Right**
Diplopia is often associated with cranial nerve palsies, particularly those affecting the third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves. These nerves control the movement of the eyes, and damage to them can lead to diplopia. For example, a third cranial nerve palsy can cause the eye to turn outward and downward, resulting in diplopia. The correct answer is a condition that is known to cause diplopia due to a cranial nerve palsy.

**Why Each Wrong Option is Incorrect**
**Option A:** Myasthenia gravis is a neuromuscular disorder that affects the transmission of nerve impulses to muscles, including those that control eye movement. While it can cause double vision, it is not typically associated with cranial nerve palsies.
**Option B:** Stroke can cause diplopia by damaging the brain areas responsible for eye movement, but it is not as specific a cause as a cranial nerve palsy.
**Option C:** Nystagmus is an involuntary movement of the eyes, but it does not typically cause diplopia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that diplopia can be a symptom of a cranial nerve palsy, particularly the third, fourth, or sixth cranial nerves. When evaluating a patient with diplopia, it's crucial to perform a thorough neurological examination, including a cranial nerve examination, to identify the underlying cause.

**Correct Answer: C. Nystagmus.**