I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 27-year-old woman is brought to the emergency department because of a 2-week history of double vision. Neurologic examination shows that the left eye does not adduct past the midline on horizontal gaze when the patient looks to the right. Leftward horizontal gaze is normal. This patient’s ocular movement deficit is most likely caused by damage to which of the following structures?
(A) Left abducens nerve
(B) Left medial longitudinal fasciculus
(C) Left nucleus of the abducens nerve
(D) Right abducens nerve
(E) Right medial longitudinal fasciculus
(F) Right nucleus of the abducens nerve
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Answer
The patient’s symptoms and findings are characteristic of internuclear ophthalmoplegia (INO), a disorder of horizontal gaze caused by a lesion in the medial longitudinal fasciculus (MLF). Now, we must choose between only 2 options, if it the right or the left. The left eye does not adduct past the midline when the patient looks to the right. This means the problem lies in coordinating the left eye’s adduction during rightward gaze. The left medial rectus (responsible for adduction) is not receiving input from the contralateral (right) abducens nucleus. This occurs because the left MLF is damaged.
The correct answer is B