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An 18-year-old girl presents with complaints of easy fatiguability and severe headaches. She also complains of irregular and sporadic periods. During the examination, you notice behavioural traits that suggest a deterioration in peripheral vision. An eye exam confirms bitemporal hemianopia. Magnetic resonance imaging of the brain reveals a large non-functional pituitary tumour. Which structure does the tumour exert pressure on to cause her visual symptoms?