A 40 year old male presented to his physician with a history of back pain that staed 2 weeks previously. The patient stated that while he was cleaning the garden he felt a little pain in his lower back. Over the past few days he noticed that pain had got worse. He had stiffness of his back & could not sit for longer periods. Walking seemed to help. Whenever he coughed or sneezed, the pain would shoot down his right leg. Physical examination revealed that he was in moderate distress. He has spasm and tenderness of the paraspinal muscles on the right lumbar region. Straight leg raising test was 500 on the right, but full on the left. Additional examination of the right foot revealed weak dorsiflexion and hypesthesia over the first web space. The rest of the neurological examination was normal. What is the most likely reason for his symptoms?