A 78 year old man is admitted to A&E with sudden onset of right arm and leg weakness associated with problems with his vision and speech which has been present for the last 6 hours. On examination he has flaccid weakness in both right limbs and a right sided homonymous hemianopia. He has a background of hypertension, diabetes, and ischaemic heart disease. Which lesion could account for his visual symptoms (see picture below)?