Neurodegenerative Diseases USMLE Step 1 Practice Question
A 60-year-old man with longstanding type 2 diabetes mellitus (HbA1c 8.2%) presents with sudden-onset diplopia. Vital signs: BP 148/92, HR 82, RR 16, Temp 37°C, SpO2 98%. Examination reveals the left eye positioned down and out with ptosis; importantly, the pupil remains equal and reactive to light bilaterally. MRI brain shows no acute infarction. He denies headache or neck stiffness. Which mechanism best explains the preserved pupillary function?
Answer choices
- AThe lesion is in the Edinger Westphal nucleus only
- BThe trochlear nerve is compensating for the deficit
- CThe optic nerve is intact
- DParasympathetic fibers run superficially and are spared in ischemic injuryCorrect answer
- EThe sympathetic chain is also damaged
- FF. The oculomotor nerve nuclei are located in the midbrain dorsal surface away from vascular territory
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.