A 62-year-old man presents to the emergency department 2 hours after sudden onset of numbness and tingling in his left arm and leg. He reports difficulty identifying the position of his left toes when his eyes are closed and has trouble distinguishing sharp vs dull sensation on the left side of his body below the neck. Strength is 5/5 throughout, facial sensation is intact, and cranial nerves are normal. MRI brain shows a small lacunar infarct involving a right thalamic relay nucleus that receives ascending input from both the dorsal column–medial lemniscus and spinothalamic tracts and projects to the primary somatosensory cortex.

Which thalamic nucleus is most likely affected?