Which cerebral artery or branch is typically affected in a patient presenting with isolated facial weakness, and on which side?

Facial weakness is most commonly associated with an infarct in the territory of the middle cerebral artery (MCA), particularly affecting the lateral aspect of the primary motor cortex. This is often on the contralateral side to the facial weakness observed. Key diagnostic features include sudden onset of facial droop, often accompanied by arm weakness, and sparing of the forehead due to bilateral cortical representation. A common pitfall is confusing this with Bell's palsy, which involves the entire side of the face and is peripheral in origin.