## **Core Concept**
One and a half syndrome, also known as internuclear ophthalmoplegia (INO), is a disorder of eye movement characterized by impaired adduction (movement towards the nose) of one eye and nystagmus of the contralateral abducting eye. This condition typically results from a lesion in the medial longitudinal fasciculus (MLF) on one side of the brainstem.

## **Why the Correct Answer is Right**
In one and a half syndrome, the only intact eye movement is the **abduction of the eye on the side opposite to the lesion**. This is because the lesion affects the MLF, disrupting the internuclear connections necessary for conjugate eye movements, particularly adduction. The abducens nucleus and its connections, which control abduction, are typically spared. Therefore, the eye on the side of the lesion cannot adduct, and the contralateral eye can only abduct.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because adduction of the eye on the side of the lesion is impaired due to the MLF lesion.
- **Option B:** Incorrect because, although abduction of the eye on the side of the lesion might seem plausible, it's actually the abduction of the contralateral eye that's typically spared and observed.
- **Option C:** Incorrect because convergence, which involves inward movement of both eyes, can be preserved in some cases of INO but is not the only intact movement; it does not directly relate to the specific pattern of eye movement abnormalities seen in one and a half syndrome.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature to remember is that in one and a half syndrome, the **abduction of the contralateral eye** is the only horizontal eye movement that remains intact. This condition often results from multiple sclerosis or stroke affecting the brainstem. Clinicians should also be aware that while INO can present with nystagmus in the abducting eye, the primary issue is the inability to adduct the eye on the same side as the lesion.

## **Correct Answer:** . Abduction of contralateral eye