EM@3AM: Cortical Hand-Knob Stroke Mimicking Peripheral Nerve Injury

A 95-year-old man presents after waking with sudden inability to move the fingers and thumb of his right hand. He can lift his arm, flex and extend his wrist normally, and reports intact sensation. He denies numbness, facial droop, dysarthria, headache, gait disturbance, chest pain, or dyspnea. He has 0/5 strength in finger flexion and extension as well as in the intrinsic hand musculature, with preserved proximal upper extremity strength and intact sensation. He has no other appreciable deficits and normal vital signs. What is the diagnosis?