A 30-year-old female is admitted
for investigation of headache. On examination of her pupils you notice that
her left pupil is semi-dilated and reacts slowly to both light and
accommodation. Her visual acuity is normal, there is no field defect, no
ptosis and eye movements are normal with no nystagmus or diplopia. Examination
of the fundus does not reveal papilloedema. The pupillary abnormality in this
patient is due to:
