Neuroanatomy USMLE Step 1 Practice Question

A 29-year-old man with no significant past medical history presents to the emergency department with 3 days of progressive weakness starting in both feet and ascending toward his trunk. He reports mild paresthesias in his fingers bilaterally but denies vision changes, facial weakness, or dysphagia. His mother mentions he had an upper respiratory infection 2 weeks ago that resolved without complications. Vital signs: BP 118/76, HR 92, RR 18, SpO2 98% on room air. Neurologic examination reveals 2/5 strength in bilateral lower extremities with absent patellar and Achilles reflexes; upper extremity strength is 4/5 with diminished but present reflexes. Sensory examination to light touch is intact. Electromyography shows acute denervation potentials and fibrillation in lower extremity muscles with normal sensory nerve conduction velocities. Lumbar cerebrospinal fluid shows protein 68 mg/dL (normal <45), normal glucose, and normal cell count. Which anatomical structure contains the neuronal cell bodies responsible for the acute muscle denervation observed on electromyography?