A 58-year-old man with hypertension presents with acute-onset left eye ptosis, impaired adduction and depression of the left eye, and a fixed dilated left pupil. Fundoscopy reveals no retinal changes. The structure marked **C** in the Circle of Willis diagram is most likely affected. Which clinical finding best explains why the pupil is involved in this presentation, distinguishing it from diabetic CN III palsy?

Explanation

## Why option 3 is right The Posterior Communicating Artery (PComm) runs directly adjacent to the oculomotor nerve (CN III) as it passes through the interpeduncular cistern. A PComm aneurysm causes mass effect and compression of CN III, affecting all fiber populations—both motor fibers (causing the "down and out" eye with ptosis, impaired adduction and depression) and parasympathetic fibers (causing mydriasis and loss of pupillary light reflex). The key distinguishing feature is that compressive lesions affect the entire nerve cross-section, including the parasympathetic fibers that run throughout the nerve. This contrasts sharply with ischemic CN III palsy (as in diabetes), where the small penetrating vessels supplying the nerve core are occluded, sparing the parasympathetic fibers that run peripherally and have better collateral supply. Gray's Anatomy 42e emphasizes the anatomical relationship between PComm and CN III as the basis for this classic clinical syndrome. ## Why each distractor is wrong - **Option 1**: Reverses the anatomical principle. While parasympathetic fibers do run peripherally, a compressive lesion (aneurysm) affects the entire nerve including these peripheral fibers, causing mydriasis. The statement incorrectly suggests compression spares the pupil. - **Option 2**: Confuses the pathophysiology. This describes diabetic (ischemic) CN III palsy correctly—pupil is spared in ischemia because parasympathetic fibers are on the periphery with good collateral supply. However, the question stem describes a compressive lesion (PComm aneurysm), not ischemia, so this is the wrong mechanism for the clinical presentation. - **Option 4**: Misattributes the cause to hypertension-induced ischemia rather than mechanical compression by the aneurysm. Hypertension alone does not selectively damage parasympathetic fibers; the fixed dilated pupil here results from compression. **High-Yield:** PComm aneurysm → compressive CN III palsy with **pupil involved** (mydriasis); Diabetic CN III palsy → ischemic, **pupil spared** (parasympathetic fibers peripheral, collateral-rich). [cite: Gray's Anatomy 42e, Chapter 28; Neuroradiology/Neurosurgery texts on Circle of Willis aneurysms and CN III compression syndromes]