Neuroanatomy: trochlear nerve: 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: trochlear nerve

Sample item

A midbrain haemorrhage destroys the right trochlear nucleus. Which superior oblique is paralysed, and why?

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

The answer and the reasoning

D. The left superior oblique, because trochlear fibres decussate before leaving the dorsal midbrain

The trochlear nerve is the only cranial nerve that exits the dorsal brainstem and its fibres decussate in the anterior medullary velum, so each nucleus supplies the opposite superior oblique. It also has the longest intracranial course, making it vulnerable to head trauma. A lesion of the nerve itself, after the decussation, causes ipsilateral palsy.

Option by option

  • A. Both superior obliques, because the nucleus is a midline structure

    Each nucleus supplies one muscle on the opposite side, not both.

  • B. The right superior oblique, because the fibres leave on the same side

    A nuclear lesion does not weaken the ipsilateral muscle because the fibres cross.

  • C. The left inferior oblique, because the fibres join the oculomotor nerve

    The trochlear nerve supplies only the superior oblique, not the inferior oblique.

  • D. The left superior oblique, because trochlear fibres decussate before leaving the dorsal midbrain (correct)

    Correct. The decussation means a nuclear lesion affects the contralateral muscle.

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.

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Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is D, The left superior oblique, because trochlear fibres decussate before leaving the dorsal midbrain. The trochlear nerve is the only cranial nerve that exits the dorsal brainstem and its fibres decussate in the anterior medullary velum, so each nucleus supplies the opposite superior oblique.

Why is "Both superior obliques, because the nucleus is a midline structure" not the answer?

Each nucleus supplies one muscle on the opposite side, not both.

Why is "The right superior oblique, because the fibres leave on the same side" not the answer?

A nuclear lesion does not weaken the ipsilateral muscle because the fibres cross.

Why is "The left inferior oblique, because the fibres join the oculomotor ner…" not the answer?

The trochlear nerve supplies only the superior oblique, not the inferior oblique.

What concept does this question test?

It tests Neuroanatomy: trochlear nerve in Anatomy. It also has the longest intracranial course, making it vulnerable to head trauma. A lesion of the nerve itself, after the decussation, causes ipsilateral palsy.

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: trochlear nerve?

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.