new_n_lost
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A 46-year-old woman presents to her physician with "double vision" and is unable to adduct her right eye on attempted left lateral gaze. Convergence is intact. Both direct and consensual light reflexes are normal.
Which of the following structures is most likely to be affected?
A. Left oculomotor nerve
B. Medial longitudinal fasciculus
C. Right abducens nerve
D. Right oculomotor nerve
E. Right trochlear nerve
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| acintya
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B
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| docdoc9
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B
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| new_n_lost
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acintya wrote:
B
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"never argue with a fool, they'll bring you down to their level and beat you with experience" FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| new_n_lost
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Very Important Explanation Make notes of it if you have to
The correct answer is B.
This patient is suffering from internuclear ophthalmoplegia (INO), which is caused by a lesion of the medial longitudinal fasciculus (MLF). The medial longitudinal fasciculus (MLF) connects the oculomotor (III), trochlear (IV), and abducens (VI) nuclei and is essential for conjugate gaze. A lesion in the MLF will result in the inability to medially rotate the ipsilateral eye on attempted lateral gaze. However, a lesion of the motor fibers of the right oculomotor nerve would also lead to the same symptoms. The way to truly distinguish between an INO from a lesion of the medial rectus muscle or a lesion of the motor fibers of CN III is to determine whether the patient can converge her eyes. If the innervation of the medial rectus muscle is interrupted, the patient will not be able to move the ipsilateral eye medially for either conjugate or dysconjugate (convergence) movements. However, if the lesion is in the MLF, this would only affect conjugate movement, and not convergence, asin this patient.
The left oculomotor nerve (choice A) is intact because the light reflexes are normal, and there is no description of any eye movement disorders of the left eye.
A lesion of the right abducens nerve (choice C) would result in paralysis of the lateral rectus muscle. This would lead to an inability to abduct the right eye.
The right oculomotor nerve (choice D) innervates the medial rectus muscle, which would lead to an inability to adduct the right eye when looking toward the left. (CN III also innervates the superior rectus, inferior rectus, and inferior oblique extraocular muscles.) However, a patient with a lesion of this nerve would also be unable to converge. A patient with a lesion of CN III would also be expected to have the affected eye look "down and out" because of the unopposed actions of the lateral rectus and superior oblique muscles, and ptosis resulting from denervation of the levator palpebrae muscle. Additionally, a lesion of the right oculomotor nerve could affect light reflexes and produce mydriasis if the parasympathetic fibers of this nerve were damaged.
The trochlear nerve (choice E) innervates the superior oblique muscle, which depresses, intorts, and abducts the eye. A lesion of this nerve theoretically could produce a slight extorsion of the eye and a weakness of downward gaze. However, a lesion of this nerve often produces only minimal symptoms because of the preserved action of muscles that are innervated by the unaffected oculomotor nerve, and because of a compensatory head tilt.
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"never argue with a fool, they'll bring you down to their level and beat you with experience" FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| ieltsforall
Forum Junior
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B
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