A 63-year-old man with long-standing hypertension and hyperlipidemia develops sudden-onset vertigo, nausea, hoarseness, and difficulty swallowing. On examination, he has decreased pain and temperature sensation on the left side of his face and the right side of his body. He also has left-sided limb ataxia and a leftward deviation of the uvula. MRI shows an infarct involving the lateral aspect of the medulla.

Occlusion of which artery most likely caused this patient's stroke?