A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 56-year-old man comes to the physician because of worsening double vision and drooping of the right eyelid for 2 days. He has also had frequent headaches over the past month. Physical examination shows right eye deviation laterally and inferiorly at rest. The right pupil is dilated and does not react to light or with accommodation. The patient's diplopia improves slightly on looking to the right. Which of the following is the most likely cause of this patient’s findings?
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A
Aneurysm of the posterior communicating arteryCorrect. A PCom artery aneurysm compresses the adjacent oculomotor nerve, classically presenting with a painful 'down-and-out' eye plus a fixed, dilated pupil — the cardinal compressive CN III lesion.
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B
Demyelination of the medial longitudinal fasciculusIncorrect. MLF demyelination causes internuclear ophthalmoplegia (impaired adduction with contralateral abducting nystagmus), not a fixed dilated pupil or ptosis.
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C
Enlarging pituitary adenomaIncorrect. Pituitary adenomas extending laterally into the cavernous sinus typically affect multiple cranial nerves (III, IV, V1, V2, VI) and present with bitemporal hemianopia from chiasmal compression, not isolated pupil-involving CN III palsy.
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D
Infarction of the midbrainIncorrect. Midbrain infarction causes a CN III palsy plus crossed motor or cerebellar findings (Weber or Benedikt syndrome); an isolated, pupil-involving CN III with headache is more typical of aneurysm.
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E
Compression of the abducens nerve at Dorello canalIncorrect. CN VI compression causes isolated horizontal diplopia worse on lateral gaze with medial deviation of the eye — opposite of this patient's down-and-out eye with ptosis and a fixed dilated pupil.
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Answer: A. A 56-year-old man with subacute headaches, ptosis, and an eye deviated 'down and out' with a fixed, dilated pupil has a complete third (oculomotor) nerve palsy with pupillary involvement. The pupillary fibers run on the dorsomedial surface of CN III and are the first compressed by an extrinsic mass — most classically an aneurysm of the posterior communicating artery (PCA-junction or PCom aneurysm), which sits directly adjacent to the nerve as it exits the midbrain.
The critical clinical pearl is the 'rule of the pupil': compressive CN III palsies (aneurysm, uncal herniation, tumor) involve the pupil early, while ischemic CN III palsies (diabetes, hypertension) typically spare the pupil because the central motor fibers are vulnerable to microvascular ischemia but the peripheral parasympathetic fibers are not. A blown pupil with CN III palsy is a neurosurgical emergency — PCom aneurysms can rupture, causing subarachnoid hemorrhage.
Urgent CT angiography or MR angiography is indicated, followed by endovascular coiling or surgical clipping.
**Why each option:**
**A.** Correct. A PCom artery aneurysm compresses the adjacent oculomotor nerve, classically presenting with a painful 'down-and-out' eye plus a fixed, dilated pupil — the cardinal compressive CN III lesion.
**B.** MLF demyelination causes internuclear ophthalmoplegia (impaired adduction with contralateral abducting nystagmus), not a fixed dilated pupil or ptosis.
**C.** Pituitary adenomas extending laterally into the cavernous sinus typically affect multiple cranial nerves (III, IV, V1, V2, VI) and present with bitemporal hemianopia from chiasmal compression, not isolated pupil-involving CN III palsy.
**D.** Midbrain infarction causes a CN III palsy plus crossed motor or cerebellar findings (Weber or Benedikt syndrome); an isolated, pupil-involving CN III with headache is more typical of aneurysm.