A 63-year-old man is brought to the emergency department 90 minutes after sudden onset of slurred speech and right-sided weakness. He has a history of long-standing hypertension and type 2 diabetes mellitus. On exam, he has right facial droop and right arm weakness greater than leg weakness. Noncontrast CT head shows no hemorrhage. An axial MRI (horizontal section) performed shortly thereafter demonstrates a small acute infarct involving a compact white matter structure that lies between the head of the caudate nucleus medially and the lentiform nucleus laterally; the lesion is centered at the “bend” where anterior and posterior limbs meet.

Which structure is most likely involved?

(Answer choices refer to structures commonly labeled on an axial MRI through the basal ganglia and diencephalon.)