A 72-year-old man with a history of poorly controlled hypertension presents to the emergency department with acute-onset right facial droop, right arm weakness, and slurred speech. His wife reports symptom onset 90 minutes ago. Vital signs show BP 168/92 mmHg, HR 88 bpm, RR 16, temperature 37°C, and SpO2 98% on room air. Neurologic examination reveals right-sided facial weakness sparing the forehead, right arm grade 3/5 weakness, dysarthria, and intact sensation. Brain MRI with diffusion-weighted imaging demonstrates an acute ischemic infarct in the left middle cerebral artery distribution involving the left motor cortex and internal capsule. Which of the following anatomical structures is primarily responsible for the contralateral (right-sided) motor deficits observed in this patient?

1. A)Decussation of corticospinal fibers in the medullary pyramidsGABARITO
2. B)Crossing of uncrossed corticobulbar tract fibers at the facial motor nucleus
3. C)Ipsilateral corticospinal tract descending through the brainstem
4. D)Decussation of spinocerebellar fibers in the lower medulla
5. E)Crossing of corticopontine fibers within the pontine nuclei