What is causing your knee pain when you run and how serious is it?

Today on the Ask Coach Parry Podcast we answer a question relating to knee pain. Nonkululo Mcuntula is hoping to run the Comrades Marathon in 2015 and had the following to say: ‘Hi coach. I have an extreme pain in my right knee when I start running downhill in a marathon. I struggled with swollen knee right through the finish, I went to see the doctor and he prescribed some steroid injections and said it is just a ligament and would go away.’
Nonkululo says that he rested it for 4 weeks but now the pain is waking him at night. He wanted to start training for the Comrades Marathon but it is really really sore.
He thought the doctor would recommend an x-ray, what would you recommend?