A 58-year-old man from rural Maharashtra presents to the emergency department with acute onset weakness of the right arm and leg that began 2 hours ago while he was working in his field. On examination, he has right-sided hemiparesis affecting the face, arm, and leg equally, with hyperreflexia and an extensor plantar response on the right. Speech is intact and there is no visual field defect. CT head shows a hyperdense lesion in the left basal ganglia region. Which structure is most likely damaged, given the pattern of motor deficit involving the face, arm, and leg in a contiguous distribution?

Explanation

## Anatomical Basis of the Deficit The patient presents with **contiguous motor weakness** affecting the face, arm, and leg on the right side (contralateral to the left-sided lesion). This pattern is pathognomonic for a lesion affecting the **corticospinal tract**, which passes through the posterior limb of the internal capsule. ### Internal Capsule Organization | Limb | Contents | Clinical Deficit if Damaged | |------|----------|-----------------------------| | **Anterior** | Thalamocortical radiations (frontal lobe), frontopontine fibres | Personality changes, cognitive deficits, dysarthria | | **Genu** | Corticobulbar fibres (CN motor nuclei) | Facial weakness, dysarthria, dysphagia | | **Posterior** | Corticospinal tract, sensory radiations | Contralateral hemiparesis (face, arm, leg), hemisensory loss | | **Retrolenticular** | Optic radiations | Contralateral homonymous hemianopia | ### Why Posterior Limb? **Key Point:** The corticospinal tract (pyramidal tract) descends from the motor cortex → internal capsule (posterior limb) → cerebral peduncle → pons → medullary pyramids → decussation → spinal cord. A single lesion in the posterior limb produces **contiguous motor loss** of the entire contralateral body. **Clinical Pearl:** The "pure motor stroke" pattern (face + arm + leg weakness without sensory loss or visual field defect) is the classic presentation of posterior limb internal capsule infarction, typically from lacunar stroke in hypertensive patients. **High-Yield:** The posterior limb is the most commonly affected region in lacunar stroke due to its vascular territory (lenticulostriate arteries from the middle cerebral artery). [cite:Snell's Neuroanatomy 8e Ch 10]