Free COMLEX Level 1 Sample Questions

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Your Free COMLEX Level 1 Practice Question

A 30-year-old female presents to the emergency department with complaint of diplopia when she looks downward, especially when descending stairs. History reveals that the diplopia began after falling and hitting the back of her head while at work earlier that day. Ocular motility testing reveals the right eye is rotated slightly upward relative to the left eye. MRI reveals a lesion just caudal to the left inferior colliculus. 

Which of the following extraocular eye muscles is most likely paralyzed as the result of this trauma?

• A. inferior oblique
• B. inferior rectus
• C. lateral rectus
• D. superior oblique
• E. superior rectus

Answer A: The oculomotor nerve (cranial nerve III) emerges anteriorly from the brainstem medial to the crus cerebri before coursing through the cavernous sinus and into the orbit. The oculomotor nerve innervates the inferior oblique, levator palpebrae superioris, and the superior, middle, and inferior rectus muscles. Injuries to this nerve result in lateral strabismus (an eye that is abducted and depressed), ptosis (drooping of the eyelid), a dilated pupil, and loss of accommodation.

Answer B: The oculomotor nerve (cranial nerve III) emerges anteriorly from the brainstem medial to the crus cerebri before coursing through the cavernous sinus and into the orbit. The oculomotor nerve innervates the inferior rectus, superior rectus, medial rectus, and the levator palpebrae superioris. Injuries to this nerve result in lateral strabismus (an eye that is abducted and depressed), ptosis (drooping of the eyelid), a dilated pupil and loss of accommodation.

Answer C: The abducens nerve (cranial nerve VI) emerges anteriorly from the brainstem between the pons and medulla before coursing through the cavernous sinus and into the orbit. The abducens nerve innervates the lateral rectus muscle, which abducts the eye. Lesions of this nerve result in medial strabismus (an eye that is adducted).

Answer E: Inhibition of metarhodopsin II formation would occur in the absence of light. After light converts 11-cis rhodopsin to all-trans rhodopsin, a series of conformational changes occur in the opsin that eventually produces metarhodopsin II. This would cause a complete lack of vision rather than cyanopsia, as seen in this patient.

Bottom Line: Lesions of the trochlear nerve (cranial nerve IV) result in dysfunction of the superior oblique muscle and an inability to depress the eye, leading to diplopia when the patient tries to look downward.

For more information, see:

• Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. 5th Ed. Philadelphia, PA: Elsevier; 2017:184-185, 483-484.
• Fourth Cranial Nerve (Trochlear Nerve) Palsy on UpToDate
• First Aid for the USMLE Step 1 2019: 528-529, 494.
• First Aid for the USMLE Step 1 2020: 540-543, 506.
• First Aid for the USMLE Step 1 2021: 560-561.
• First Aid for the USMLE Step 1 2022: 560-561.
• First Aid for the USMLE Step 1 2023: 520-521.
• First Aid for the USMLE Step 1 2024: 518, 519
• First Aid for the USMLE Step 1 2025: 555, 556.

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