A 72-year-old woman with a history of hypertension and previous stroke presents with a 3-day history of right-sided headache and diplopia. On examination, the right pupil is 5 mm and reacts sluggishly to light, while the left pupil is 3 mm and reacts briskly. The right eye is adducted and depressed at rest. Visual acuity is 6/6 in both eyes. Fundoscopy is normal. MRI brain with contrast shows an acute infarct in the right midbrain. What is the most likely pupillary abnormality?