A 60 year old man, presented with a 3 month history of progressive left leg weakness. He has type 2 diabetes mellitus but is otherwise fit and well. On examination of the lower limbs: inspection is normal, tone normal, left leg power is grade 1/5 and right leg power 5/5. On examination of the reflexes the left leg reflexes are slightly brisker than the right. On sensory examination there is reduced sensation to the coldness of the tuning fork and pin prick in the right leg. Where is the lesion?