Neuroanatomy: internuclear ophthalmoplegia: practice question

A Anatomy question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anatomy · Neuroanatomy: internuclear ophthalmoplegia

Sample item

A 28-year-old woman with multiple sclerosis cannot adduct her left eye on looking to the right. The right eye abducts with nystagmus. Convergence is preserved. Where is the lesion?

ALeft oculomotor nerve in the subarachnoid spaceBLeft abducens nucleusCRight medial longitudinal fasciculus in the ponsDLeft medial longitudinal fasciculus in the pons

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Left medial longitudinal fasciculus in the pons

On right gaze, the right abducens nucleus drives the right lateral rectus and sends internuclear neurons across the midline into the left medial longitudinal fasciculus, to the left oculomotor nucleus which supplies the left medial rectus. A lesion of the left MLF produces failure of left adduction, with abducting nystagmus of the right eye. Convergence is spared because it bypasses the MLF. Demyelination is a classic cause in young adults.

Option by option

A. Left oculomotor nerve in the subarachnoid space
A left oculomotor nerve lesion would also impair convergence and cause ptosis and a dilated pupil.
B. Left abducens nucleus
A left abducens nucleus lesion causes gaze palsy to the left, not failure of left adduction.
C. Right medial longitudinal fasciculus in the pons
A right MLF lesion would cause failure of right adduction on left gaze.
D. Left medial longitudinal fasciculus in the pons (correct)
A left MLF lesion produces left adduction failure on right gaze, with preserved convergence.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is D, Left medial longitudinal fasciculus in the pons. On right gaze, the right abducens nucleus drives the right lateral rectus and sends internuclear neurons across the midline into the left medial longitudinal fasciculus, to the left oculomotor nucleus which supplies the left medial rectus.

Why is "Left oculomotor nerve in the subarachnoid space" not the answer?

A left oculomotor nerve lesion would also impair convergence and cause ptosis and a dilated pupil.

Why is "Left abducens nucleus" not the answer?

A left abducens nucleus lesion causes gaze palsy to the left, not failure of left adduction.

Why is "Right medial longitudinal fasciculus in the pons" not the answer?

A right MLF lesion would cause failure of right adduction on left gaze.

What concept does this question test?

It tests Neuroanatomy: internuclear ophthalmoplegia in Anatomy. A lesion of the left MLF produces failure of left adduction, with abducting nystagmus of the right eye. Convergence is spared because it bypasses the MLF. Demyelination is a classic cause in young adults.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Neuroanatomy: internuclear ophthalmoplegia?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.