Left knee hurts when ugo up stairs or hills.what can be the problem?

Knee pain: Pain in the knee associated with ascending and dwscending stairs is usually due to the relationship between the kneecap (patella) and underlying thigh bone (trochlea/femur); we term this as a patellofemoral disorder and can respond to eccentric quadriceps strengthening, vmo strrengthening, short arc quads. A good physical therapist can provide the know how and skill to work with you.