A 58-year-old right-handed man presents to the emergency department with acute onset of expressive aphasia, right-sided facial droop, and right arm weakness (leg relatively spared). Non-contrast CT brain is performed urgently. The structure marked **A** in the diagram shows a hyperdense appearance relative to the surrounding brain parenchyma. Which of the following best explains this radiological finding and its clinical significance?

Explanation

## Why option 1 is correct The hyperdense MCA sign represents a visible thrombus within the M1 segment of the middle cerebral artery on non-contrast CT. This is the earliest specific radiological sign of acute MCA occlusion, appearing immediately at symptom onset before any parenchymal ischemic changes develop (which typically take 6–12 hours to become visible on NCCT). The sign has high specificity (>90%) though modest sensitivity (30–50%), and its presence is strongly associated with large vessel occlusion requiring urgent mechanical thrombectomy. The clinical presentation—expressive aphasia, right facial droop, and right arm weakness—is classic for dominant hemisphere (left) MCA territory infarction, affecting Broca area and the motor cortex. [Harrison 21e Ch 426] ## Why each distractor is wrong - **Option 2 (Hemorrhagic transformation)**: Hemorrhagic transformation occurs days after the initial ischemic stroke, not at acute onset. It would show blood within brain parenchyma or ventricles, not as a hyperdense sign within the arterial lumen itself. The clinical presentation here is acute ischemic stroke, not hemorrhage. - **Option 3 (Arterial dissection)**: While arterial dissection can cause stroke, it typically presents with a "string sign" or tapered narrowing on CTA/MRA, not a focal hyperdense thrombus within the vessel. Dissection is less common than thrombotic occlusion in acute MCA stroke and would not produce the characteristic hyperdense MCA sign. - **Option 4 (Atherosclerotic plaque with calcification)**: Chronic atherosclerotic disease with calcified plaque is a risk factor for stroke but does not produce an acute hyperdense sign within the vessel lumen. Calcification appears hyperdense but is chronic and would not explain the acute presentation. The hyperdense MCA sign specifically indicates acute thrombus, not chronic plaque. **High-Yield:** Hyperdense MCA sign = acute thrombus in M1 segment; >90% specific for LVO; earliest sign before infarct core appears; warrants immediate CTA/thrombectomy evaluation. [cite: Harrison 21e Ch 426]