A 62-year-old man presents to the emergency department 1 hour after sudden onset of left-sided arm and leg weakness. He reports that he can still feel a pinprick on both sides of his body but has difficulty walking because his left leg feels “clumsy.” Neurologic exam shows decreased strength in the left upper and lower extremities (2/5) with hyperreflexia and an upgoing plantar response on the left. Light touch, vibration, and joint position sense are intact. Facial sensation to pain and temperature is intact bilaterally. MRI demonstrates a small ischemic infarct involving the ventral midline of the caudal medulla. Which of the following structures is most likely involved at this brainstem level?

(Assume the lesion is at a level where there is no open fourth ventricle.)