Patellofemoral pain syndrome

A 16-year-old girl comes to her pediatrician’s office for evaluation of left knee pain. The pain has been present for the past 3-4 months, and worsens with running, prolonged sitting, or when going up or down steps. The patient also mentions episodes of her left knee “buckling.” She has taken ibuprofen which occasionally alleviates the pain. Vitals are within normal limits. BMI is 19.5 kg/m². Physical examination shows a normal gait. There is no visible knee deformity. When the patient is asked to perform squats, there is significant pain in the left knee. Which of the following best describes the etiology of this patient’s condition?