**Core Concept**
Bilateral internuclear ophthalmoplegia (INO) is a type of eye movement disorder characterized by impaired adduction of the affected eye and nystagmus of the abducting eye. It is often associated with damage to the medial longitudinal fasciculus (MLF), a critical brainstem structure that coordinates horizontal eye movements.

**Why the Correct Answer is Right**
The most common cause of bilateral INO is multiple sclerosis (MS), a chronic autoimmune disease that affects the central nervous system. MS can cause demyelination and damage to the MLF, leading to the characteristic symptoms of INO. The damage to the MLF disrupts the normal coordination of horizontal eye movements, resulting in impaired adduction of the affected eye and nystagmus of the abducting eye.

**Why Each Wrong Option is Incorrect**
**Option A:** While stroke is a possible cause of unilateral INO, it is not typically associated with bilateral INO. Stroke can cause damage to the MLF, but it is usually unilateral and does not typically affect both sides of the brainstem.
**Option B:** Parkinson's disease is a neurodegenerative disorder that affects motor control, but it is not typically associated with INO. Parkinson's disease can cause a range of eye movement abnormalities, but INO is not a characteristic feature.
**Option C:** Progressive supranuclear palsy (PSP) is a neurodegenerative disorder that affects eye movement, but it is not typically associated with INO. PSP can cause a range of eye movement abnormalities, including difficulty with vertical eye movements, but INO is not a characteristic feature.

**Clinical Pearl / High-Yield Fact**
Bilateral INO is often associated with a "one-and-a-half" syndrome, where the affected eye is unable to adduct and the abducting eye has nystagmus. This clinical presentation is highly suggestive of MS or other demyelinating diseases that affect the MLF.

**Correct Answer:** C. Multiple sclerosis.