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Comprehensive and Detailed Explanation:

The correct answer is B because the internal capsule—specifically the posterior limb—contains densely packed descending corticospinal tract fibers originating from the motor cortex. When the middlecerebral artery (MCA) or its deep penetrating branches (lenticulostriate arteries) are occluded, ischemia involving the internal capsule leads to dense contralateral hemiparesis or hemiplegia. Option A is incorrect because the thalamus acts primarily as a major sensory relay station, though thalamic strokes produce profound sensory loss. Option C is incorrect because the substantia nigra is located in the midbrain and is primarily dopaminergic, associated with Parkinsonian features rather than acute MCA stroke hemiparesis. Option D is incorrect because the corpus callosum connects the two cerebral hemispheres and is supplied by the pericallosal branches of the anterior cerebral artery. Mastering the micro-anatomy of white matter tracts like the internal capsule allows SCRN nurses to accurately predict motor and sensory deficits from vascular lesions.