What causes pain and burning down the front of the left upper thigh and leg, right above knee?

Depends on your His-: Tory, if any injury or past hx of it. One of the diseases (with type of symptoms like the one you have), is called meralgia paresthetica. With pain/tingling over the front ; outer aspect of the thigh, from being pinched in the area of the outer end of the groin crease. Pain meds + stretching exercises are helpful. See your orthopedist.